feat: packed single-file embedding index (closes #3)

The semantic index was ~90 896 per-entry files (index/embeddings/<safe-model>/
<sha256>.f32), a deployment liability: the file count — not the 356 MB — is
hostile to container layers, object storage, and Git-LFS, and Pipeline::load
opened every one individually. This packs a complete per-file store into one
self-describing matrix file loaded in a single read.

New src/packed.rs owns the format: 4-byte magic "AIPK", u32 LE format_version,
u32 LE header length, a JSON PackedHeader {n_rows, dim, embed_model,
corpus_sha256, corpus_name}, pad to a 4-byte boundary, then row-major
little-endian f32 payload in corpus order (row i = corpus entry i). write_packed
is atomic (temp+rename); read_packed rejects a wrong magic, a newer-than-
supported version, a malformed header, or a payload length inconsistent with
n_rows×dim. corpus_sha256 hashes the raw corpus file bytes.

Pipeline::load now calls a free fn load_packed_or_store(cfg, entries) that
prefers the packed file and falls back to today's all-or-nothing per-file store,
reporting which path it took via a new IndexStatus {Ok, Absent, Mismatch}
carried into Diagnostics. `alpha-id index --pack` writes the file from a
complete store (refusing an incomplete one with exit 2), recording corpus_sha256
+ embed_model in the header as the artifact-binding data.

Iteration boundary (deliberate): this is iteration 1 of the packed-versioned-
bundle cycle. load_packed_or_store validates the packed file STRUCTURALLY only
(magic/version/payload length via read_packed, plus n_rows == entries.len());
it does NOT yet compare the header's corpus_sha256/embed_model against the live
config, and IndexStatus::Mismatch fires only on structural corruption. The
load-time mismatch warning, the index= token on the diagnostics line, and the
Hybrid-degrade-names-reason wiring are iteration 2 (issue #4), which consumes
the binding data this commit writes. The per-file store is untouched — it stays
the build/resume path; the packed file is an additive serve-path artifact.

mmap was considered and rejected: the brute-force full-scan cosine search
touches every row per query, so memory-mapping yields no runtime benefit over a
single read and would add an unsafe dependency (spec § Out of scope).

Verification: cargo test green (61 tests; 9 new — 3 format round-trip/hash/bad-
magic, 4 load-path incl. packed-beats-store and structural n_rows mismatch, 2
--pack CLI incl. exit-2 refusal). An end-to-end seam test writes the packed
file via the CLI, removes the per-file store, and asserts the pipeline builds
its index from the packed file alone as IndexStatus::Ok (#3 acceptance b).

Spec: docs/specs/2026-05-31-packed-versioned-index-bundle.md
Plan: docs/plans/2026-06-01-packed-versioned-bundle-iter1.md

closes #3

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/bin/alpha_id.rs

@@ -15,7 +15,7 @@ struct Cli {
 #[derive(Subcommand)]
 enum Cmd {
     /// Build indexes (lexical now; embeddings added in Phase A)
-    Index { #[arg(long)] sample: Option<usize>, #[arg(long)] full: bool, #[arg(long)] confirm: bool },
+    Index { #[arg(long)] sample: Option<usize>, #[arg(long)] full: bool, #[arg(long)] confirm: bool, #[arg(long)] pack: bool },
     /// Suggest ICD codes for a dictation (FILE or - for stdin)
     Suggest {
         input: String,
@@ -80,9 +80,42 @@ fn main() {
     let cli = Cli::parse();
     let cfg = Config::load(&cli.config).expect("config");
     match cli.cmd {
-        Cmd::Index { sample, full, confirm } => {
+        Cmd::Index { sample, full, confirm, pack } => {
             use alpha_id::embed::{EmbeddingStore, build_corpus_embeddings, estimate};
             use alpha_id::ionos::IonosClient;
+
+            if pack {
+                use alpha_id::packed::{corpus_sha256, packed_path, write_packed, PackedHeader};
+                let entries = alpha_id::corpus::load(&cfg.alpha_id_path).expect("corpus");
+                let store = EmbeddingStore::open(&cfg.index_dir, &cfg.embed_model).expect("store");
+                let cached = entries.iter().filter(|e| store.has(&e.text)).count();
+                if cached != entries.len() {
+                    eprintln!(
+                        "error: store incomplete: {} of {} entries embedded; \
+                         run `index --full --confirm` first",
+                        cached, entries.len()
+                    );
+                    std::process::exit(2);
+                }
+                let rows: Vec<Vec<f32>> =
+                    entries.iter().map(|e| store.get(&e.text).expect("cached vector")).collect();
+                let dim = rows.first().map(|r| r.len()).unwrap_or(0) as u32;
+                let corpus_name = std::path::Path::new(&cfg.alpha_id_path)
+                    .file_name().and_then(|n| n.to_str())
+                    .unwrap_or(&cfg.alpha_id_path).to_string();
+                let header = PackedHeader {
+                    n_rows: entries.len() as u32,
+                    dim,
+                    embed_model: cfg.embed_model.clone(),
+                    corpus_sha256: corpus_sha256(&cfg.alpha_id_path).expect("corpus hash"),
+                    corpus_name,
+                };
+                let path = packed_path(&cfg.index_dir, &cfg.embed_model);
+                write_packed(&path, &header, &rows).expect("write packed");
+                eprintln!("packed {} rows × {} dims → {}", header.n_rows, header.dim, path.display());
+                return;
+            }
+
             let p = Pipeline::load(&cfg).expect("load");
             let texts: Vec<String> = p.entries.iter().map(|e| e.text.clone()).collect();
             let client = IonosClient::new(&cfg.ionos_base_url, &cfg.token_path)

src/lib.rs

@@ -10,5 +10,6 @@ pub mod ionos;
 pub mod embed;
 pub mod vector;
 pub mod rerank;
+pub mod packed;
 pub mod pipeline;
 pub mod eval;

src/model.rs

@@ -80,6 +80,14 @@ impl Filter {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Mode { Lexical, Hybrid }
 
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Default)]
+pub enum IndexStatus {
+    Ok,               // packed file present and structurally valid
+    #[default]
+    Absent,           // no usable index — intentional lexical-only
+    Mismatch(String), // packed file present but structurally invalid; reason carried
+}
+
 impl FromStr for Mode {
     type Err = String;
     fn from_str(s: &str) -> Result<Self, String> {
@@ -118,6 +126,7 @@ pub struct Diagnostics {
     pub segment_count: usize,
     pub ionos_calls: usize,
     pub millis: u128,
+    pub index_status: IndexStatus,
 }
 
 #[derive(Debug, Serialize)]

src/packed.rs

@@ -0,0 +1,115 @@
+use crate::model::AppError;
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+use std::fs;
+use std::path::{Path, PathBuf};
+
+const MAGIC: &[u8; 4] = b"AIPK";
+pub const PACKED_FORMAT_VERSION: u32 = 1;
+
+/// Manifest written into the packed file header. `embed_model` and
+/// `corpus_sha256` bind the matrix to the corpus snapshot and model it was
+/// built from; iteration 1 records them, iteration 2 enforces them.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PackedHeader {
+    pub n_rows: u32,
+    pub dim: u32,
+    pub embed_model: String,
+    pub corpus_sha256: String,
+    pub corpus_name: String,
+}
+
+/// Derived packed-file path `<index_dir>/packed.<safe-model>.bin`, using the
+/// same `/`→`_` model sanitization `EmbeddingStore::open` applies, so `--pack`
+/// and `load` compute the identical path from the same config.
+pub fn packed_path(index_dir: &str, model: &str) -> PathBuf {
+    let safe = model.replace('/', "_");
+    PathBuf::from(index_dir).join(format!("packed.{safe}.bin"))
+}
+
+/// SHA-256 of the raw bytes of the corpus file at `path`, lower-case hex.
+pub fn corpus_sha256(path: &str) -> Result<String, AppError> {
+    let bytes = fs::read(path).map_err(|e| AppError::Io(format!("{path}: {e}")))?;
+    let mut h = Sha256::new();
+    h.update(&bytes);
+    Ok(h.finalize().iter().map(|x| format!("{:02x}", x)).collect())
+}
+
+/// Write the packed matrix: magic + format_version + header_len + JSON header
+/// + pad-to-4 + row-major little-endian f32 payload. Atomic via temp+rename.
+pub fn write_packed(path: &Path, header: &PackedHeader, rows: &[Vec<f32>]) -> Result<(), AppError> {
+    let json = serde_json::to_vec(header)
+        .map_err(|e| AppError::Parse(format!("packed header: {e}")))?;
+    let mut out = Vec::new();
+    out.extend_from_slice(MAGIC);
+    out.extend_from_slice(&PACKED_FORMAT_VERSION.to_le_bytes());
+    out.extend_from_slice(&(json.len() as u32).to_le_bytes());
+    out.extend_from_slice(&json);
+    while out.len() % 4 != 0 {
+        out.push(0);
+    }
+    for row in rows {
+        for f in row {
+            out.extend_from_slice(&f.to_le_bytes());
+        }
+    }
+    let tmp = path.with_extension("tmp");
+    fs::write(&tmp, &out).map_err(|e| AppError::Io(format!("{}: {e}", tmp.display())))?;
+    fs::rename(&tmp, path).map_err(|e| AppError::Io(format!("{}: {e}", path.display())))?;
+    Ok(())
+}
+
+/// Read a packed matrix back into its header and row vectors. Fails on a
+/// wrong magic, an unsupported (newer) format version, a malformed or
+/// truncated header, or a payload length inconsistent with `n_rows × dim`.
+pub fn read_packed(path: &Path) -> Result<(PackedHeader, Vec<Vec<f32>>), AppError> {
+    let bytes = fs::read(path).map_err(|e| AppError::Io(format!("{}: {e}", path.display())))?;
+    if bytes.len() < 12 || &bytes[0..4] != MAGIC {
+        return Err(AppError::Parse(format!("{}: bad magic", path.display())));
+    }
+    let version = u32::from_le_bytes([bytes[4], bytes[5], bytes[6], bytes[7]]);
+    if version > PACKED_FORMAT_VERSION {
+        return Err(AppError::Parse(format!(
+            "{}: unsupported packed format version {version}",
+            path.display()
+        )));
+    }
+    let header_len = u32::from_le_bytes([bytes[8], bytes[9], bytes[10], bytes[11]]) as usize;
+    let header_end = 12 + header_len;
+    if bytes.len() < header_end {
+        return Err(AppError::Parse(format!("{}: truncated header", path.display())));
+    }
+    let header: PackedHeader = serde_json::from_slice(&bytes[12..header_end])
+        .map_err(|e| AppError::Parse(format!("{}: header: {e}", path.display())))?;
+    let mut payload_start = header_end;
+    while payload_start % 4 != 0 {
+        payload_start += 1;
+    }
+    let n = header.n_rows as usize;
+    let dim = header.dim as usize;
+    let expected = n * dim * 4;
+    if bytes.len() - payload_start != expected {
+        return Err(AppError::Parse(format!(
+            "{}: payload length {} != n_rows×dim×4 {}",
+            path.display(),
+            bytes.len() - payload_start,
+            expected
+        )));
+    }
+    let mut rows = Vec::with_capacity(n);
+    let mut off = payload_start;
+    for _ in 0..n {
+        let mut row = Vec::with_capacity(dim);
+        for _ in 0..dim {
+            row.push(f32::from_le_bytes([
+                bytes[off],
+                bytes[off + 1],
+                bytes[off + 2],
+                bytes[off + 3],
+            ]));
+            off += 4;
+        }
+        rows.push(row);
+    }
+    Ok((header, rows))
+}

src/pipeline.rs

@@ -5,6 +5,7 @@ use crate::fusion::{cross_segment_dedupe, rrf_merge};
 use crate::ionos::IonosClient;
 use crate::lexical::LexicalIndex;
 use crate::model::*;
+use crate::packed;
 use crate::rerank;
 use crate::segment::split_segments;
 use crate::tags::tags_for;
@@ -20,6 +21,49 @@ pub struct Pipeline {
     pub valid_by_code: HashMap<String, bool>,
     cfg: Config,
     vector: Option<VectorIndex>,
+    index_status: IndexStatus,
+}
+
+/// Build the semantic index, preferring the packed matrix file and falling
+/// back to the per-file store. Returns the index (`None` when none is usable)
+/// and an `IndexStatus` describing the path taken.
+///
+/// Iteration-1 scope: the packed file is validated *structurally* only
+/// (`read_packed` checks magic/version/payload length; here we additionally
+/// require `n_rows == entries.len()`). The header's `corpus_sha256` and
+/// `embed_model` are NOT yet compared against the config — that enforcement
+/// is iteration 2.
+pub fn load_packed_or_store(cfg: &Config, entries: &[AlphaIdEntry]) -> (Option<VectorIndex>, IndexStatus) {
+    let packed = packed::packed_path(&cfg.index_dir, &cfg.embed_model);
+    if packed.exists() {
+        match packed::read_packed(&packed) {
+            Ok((header, rows)) => {
+                if header.n_rows as usize != entries.len() {
+                    return (
+                        None,
+                        IndexStatus::Mismatch(format!(
+                            "packed n_rows {} != corpus entries {}",
+                            header.n_rows,
+                            entries.len()
+                        )),
+                    );
+                }
+                return (Some(VectorIndex::from_vectors(rows)), IndexStatus::Ok);
+            }
+            Err(e) => return (None, IndexStatus::Mismatch(format!("{e}"))),
+        }
+    }
+    // Fallback: per-file store, all-or-nothing (today's behaviour).
+    match EmbeddingStore::open(&cfg.index_dir, &cfg.embed_model) {
+        Ok(store) if entries.iter().all(|e| store.has(&e.text)) => {
+            let vecs: Option<Vec<Vec<f32>>> = entries.iter().map(|e| store.get(&e.text)).collect();
+            match vecs {
+                Some(v) => (Some(VectorIndex::from_vectors(v)), IndexStatus::Ok),
+                None => (None, IndexStatus::Absent),
+            }
+        }
+        _ => (None, IndexStatus::Absent),
+    }
 }
 
 impl Pipeline {
@@ -37,17 +81,7 @@ impl Pipeline {
             }
             m
         };
-        // Build the semantic index only if EVERY corpus entry already has a
-        // cached embedding; otherwise leave it `None` so Mode Hybrid degrades to
-        // Lexical rather than mispairing partial vectors.
-        let vector = match EmbeddingStore::open(&cfg.index_dir, &cfg.embed_model) {
-            Ok(store) if entries.iter().all(|e| store.has(&e.text)) => {
-                let vecs: Option<Vec<Vec<f32>>> =
-                    entries.iter().map(|e| store.get(&e.text)).collect();
-                vecs.map(VectorIndex::from_vectors)
-            }
-            _ => None,
-        };
+        let (vector, index_status) = load_packed_or_store(cfg, &entries);
         Ok(Self {
             entries,
             meta,
@@ -56,6 +90,7 @@ impl Pipeline {
             valid_by_code,
             cfg: cfg.clone(),
             vector,
+            index_status,
         })
     }
 
@@ -232,6 +267,7 @@ impl Pipeline {
                 segment_count: segments.len(),
                 ionos_calls,
                 millis: t0.elapsed().as_millis(),
+                index_status: self.index_status.clone(),
             },
         }
     }

tests/pack_cli_tests.rs

@@ -0,0 +1,122 @@
+use alpha_id::model::{AlphaIdEntry, Config, IndexStatus};
+use alpha_id::pipeline::load_packed_or_store;
+use std::path::Path;
+use std::process::Command;
+
+// Write a minimal config TOML pointing the corpus + index dir at temp paths.
+// The `--pack` branch only reads `alpha_id_path` (corpus) and `index_dir`
+// (store); `claml_path` etc. are present for parse but unused on this path.
+fn write_config(dir: &Path, corpus: &Path, index_dir: &Path) -> std::path::PathBuf {
+    let toml = format!(
+        r#"ionos_base_url = "http://127.0.0.1:1"
+token_path = "/nonexistent"
+embed_model = "test/model"
+rerank_model = "test/rerank"
+alpha_id_path = "{corpus}"
+claml_path = "/nonexistent.xml"
+index_dir = "{index}"
+pool_size = 10
+top_k = 10
+"#,
+        corpus = corpus.to_str().unwrap(),
+        index = index_dir.to_str().unwrap(),
+    );
+    let p = dir.join("config.toml");
+    std::fs::write(&p, toml).unwrap();
+    p
+}
+
+fn corpus_line(text: &str) -> String {
+    // Alpha-ID format: valid|alpha_id|primary|star|addon|primary2|orpha|text
+    format!("1|A0001|A00|||||{text}")
+}
+
+#[test]
+fn pack_refuses_incomplete_store_exit_2() {
+    let dir = tempfile::tempdir().unwrap();
+    let corpus = dir.path().join("corpus.txt");
+    std::fs::write(&corpus, format!("{}\n{}\n", corpus_line("alpha"), corpus_line("beta"))).unwrap();
+    let index_dir = dir.path().join("index");
+    std::fs::create_dir_all(&index_dir).unwrap();
+    let config = write_config(dir.path(), &corpus, &index_dir);
+
+    let out = Command::new(env!("CARGO_BIN_EXE_alpha-id"))
+        .args(["--config", config.to_str().unwrap(), "index", "--pack"])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2), "incomplete store must exit 2");
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(stderr.contains("store incomplete"), "stderr was: {stderr}");
+}
+
+#[test]
+fn pack_writes_file_from_complete_store() {
+    let dir = tempfile::tempdir().unwrap();
+    let corpus = dir.path().join("corpus.txt");
+    std::fs::write(&corpus, format!("{}\n{}\n", corpus_line("alpha"), corpus_line("beta"))).unwrap();
+    let index_dir = dir.path().join("index");
+    std::fs::create_dir_all(&index_dir).unwrap();
+
+    // Populate the per-file store for both entry texts so the pack succeeds.
+    let store = alpha_id::embed::EmbeddingStore::open(index_dir.to_str().unwrap(), "test/model").unwrap();
+    store.put("alpha", &[1.0f32, 0.0]).unwrap();
+    store.put("beta", &[0.0f32, 1.0]).unwrap();
+
+    let config = write_config(dir.path(), &corpus, &index_dir);
+    let out = Command::new(env!("CARGO_BIN_EXE_alpha-id"))
+        .args(["--config", config.to_str().unwrap(), "index", "--pack"])
+        .output()
+        .unwrap();
+    assert!(out.status.success(), "stderr: {}", String::from_utf8_lossy(&out.stderr));
+    let packed = index_dir.join("packed.test_model.bin");
+    assert!(packed.exists(), "packed file should be written at {}", packed.display());
+}
+
+fn entry(text: &str) -> AlphaIdEntry {
+    AlphaIdEntry {
+        alpha_id: "A0001".to_string(),
+        valid: true,
+        icd_primary: "A00".to_string(),
+        icd_star: String::new(),
+        icd_addon: String::new(),
+        icd_primary2: String::new(),
+        orpha: String::new(),
+        text: text.to_string(),
+    }
+}
+
+/// End-to-end seam: a packed file produced by the `index --pack` CLI loads back
+/// through the production `load_packed_or_store` path as `IndexStatus::Ok` with a
+/// usable index, with no per-file store consulted. This pins the cross-boundary
+/// round trip (CLI writer ↔ pipeline reader) that the two halves' unit tests
+/// each cover only one side of.
+#[test]
+fn cli_packed_file_loads_back_as_ok_index() {
+    let dir = tempfile::tempdir().unwrap();
+    let corpus = dir.path().join("corpus.txt");
+    std::fs::write(&corpus, format!("{}\n{}\n", corpus_line("alpha"), corpus_line("beta"))).unwrap();
+    let index_dir = dir.path().join("index");
+    std::fs::create_dir_all(&index_dir).unwrap();
+
+    let store = alpha_id::embed::EmbeddingStore::open(index_dir.to_str().unwrap(), "test/model").unwrap();
+    store.put("alpha", &[1.0f32, 0.0]).unwrap();
+    store.put("beta", &[0.0f32, 1.0]).unwrap();
+
+    let config = write_config(dir.path(), &corpus, &index_dir);
+    let out = Command::new(env!("CARGO_BIN_EXE_alpha-id"))
+        .args(["--config", config.to_str().unwrap(), "index", "--pack"])
+        .output()
+        .unwrap();
+    assert!(out.status.success(), "stderr: {}", String::from_utf8_lossy(&out.stderr));
+
+    // Load the CLI-written packed file through the production path. The store
+    // directory is removed first to prove the index comes from the packed file
+    // alone, not the per-file store.
+    std::fs::remove_dir_all(index_dir.join("embeddings")).unwrap();
+    let mut cfg = Config::load(config.to_str().unwrap()).unwrap();
+    cfg.index_dir = index_dir.to_str().unwrap().to_string();
+    let entries = vec![entry("alpha"), entry("beta")];
+    let (vector, status) = load_packed_or_store(&cfg, &entries);
+    assert!(vector.is_some(), "packed file written by the CLI must yield an index");
+    assert_eq!(status, IndexStatus::Ok);
+}

tests/packed_load_tests.rs

@@ -0,0 +1,73 @@
+use alpha_id::model::{AlphaIdEntry, Config, IndexStatus};
+use alpha_id::packed::{packed_path, write_packed, PackedHeader};
+use alpha_id::pipeline::load_packed_or_store;
+
+fn entry(text: &str) -> AlphaIdEntry {
+    AlphaIdEntry {
+        alpha_id: "X".to_string(),
+        valid: true,
+        icd_primary: "A00".to_string(),
+        icd_star: String::new(),
+        icd_addon: String::new(),
+        icd_primary2: String::new(),
+        orpha: String::new(),
+        text: text.to_string(),
+    }
+}
+
+fn cfg_in(dir: &std::path::Path) -> Config {
+    let mut cfg = Config::load("config/default.toml").unwrap();
+    cfg.index_dir = dir.to_str().unwrap().to_string();
+    cfg.embed_model = "test/model".to_string();
+    cfg
+}
+
+#[test]
+fn index_status_defaults_to_absent() {
+    assert_eq!(IndexStatus::default(), IndexStatus::Absent);
+}
+
+#[test]
+fn index_status_serializes_externally_tagged() {
+    let j = serde_json::to_string(&IndexStatus::Mismatch("x".to_string())).unwrap();
+    assert!(j.contains("Mismatch"), "got {j}");
+}
+
+#[test]
+fn packed_beats_store_builds_without_store() {
+    let dir = tempfile::tempdir().unwrap();
+    let cfg = cfg_in(dir.path());
+    let entries = vec![entry("a"), entry("b")];
+    let rows = vec![vec![1.0f32, 0.0], vec![0.0f32, 1.0]];
+    let h = PackedHeader {
+        n_rows: 2,
+        dim: 2,
+        embed_model: cfg.embed_model.clone(),
+        corpus_sha256: "irrelevant-in-iter1".to_string(),
+        corpus_name: "c.txt".to_string(),
+    };
+    write_packed(&packed_path(&cfg.index_dir, &cfg.embed_model), &h, &rows).unwrap();
+    // No per-file store populated in `dir`; the index must come from the packed file.
+    let (vector, status) = load_packed_or_store(&cfg, &entries);
+    assert!(vector.is_some());
+    assert_eq!(status, IndexStatus::Ok);
+}
+
+#[test]
+fn n_rows_mismatch_yields_mismatch_none() {
+    let dir = tempfile::tempdir().unwrap();
+    let cfg = cfg_in(dir.path());
+    let entries = vec![entry("a"), entry("b"), entry("c")]; // 3 entries
+    let rows = vec![vec![1.0f32, 0.0], vec![0.0f32, 1.0]]; // packed claims 2 rows
+    let h = PackedHeader {
+        n_rows: 2,
+        dim: 2,
+        embed_model: cfg.embed_model.clone(),
+        corpus_sha256: "x".to_string(),
+        corpus_name: "c.txt".to_string(),
+    };
+    write_packed(&packed_path(&cfg.index_dir, &cfg.embed_model), &h, &rows).unwrap();
+    let (vector, status) = load_packed_or_store(&cfg, &entries);
+    assert!(vector.is_none());
+    assert!(matches!(status, IndexStatus::Mismatch(_)));
+}

tests/packed_tests.rs

@@ -0,0 +1,46 @@
+use alpha_id::packed::{corpus_sha256, read_packed, write_packed, PackedHeader};
+use std::io::Write;
+
+fn header(n_rows: u32, dim: u32) -> PackedHeader {
+    PackedHeader {
+        n_rows,
+        dim,
+        embed_model: "test/model".to_string(),
+        corpus_sha256: "deadbeef".to_string(),
+        corpus_name: "corpus.txt".to_string(),
+    }
+}
+
+#[test]
+fn write_read_roundtrip_preserves_header_and_rows() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("packed.bin");
+    let rows = vec![vec![0.0f32, 1.5, -2.25], vec![3.0, 4.0, 5.0]];
+    let h = header(2, 3);
+    write_packed(&path, &h, &rows).unwrap();
+    let (got_h, got_rows) = read_packed(&path).unwrap();
+    assert_eq!(got_h, h);
+    assert_eq!(got_rows, rows);
+}
+
+#[test]
+fn corpus_sha256_stable_and_sensitive() {
+    let dir = tempfile::tempdir().unwrap();
+    let p = dir.path().join("c.txt");
+    std::fs::write(&p, b"alpha|beta").unwrap();
+    let a = corpus_sha256(p.to_str().unwrap()).unwrap();
+    let b = corpus_sha256(p.to_str().unwrap()).unwrap();
+    assert_eq!(a, b, "same bytes hash identically");
+    std::fs::write(&p, b"alpha|betb").unwrap();
+    let c = corpus_sha256(p.to_str().unwrap()).unwrap();
+    assert_ne!(a, c, "one changed byte changes the hash");
+}
+
+#[test]
+fn read_packed_rejects_bad_magic() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("bad.bin");
+    let mut f = std::fs::File::create(&path).unwrap();
+    f.write_all(b"XXXX____________").unwrap();
+    assert!(read_packed(&path).is_err());
+}