iter cli-diag-human: route WorkspaceLoadError through diagnostic in non-JSON path

Closes P2 todo "CLI human-mode diagnostic surface for
WorkspaceLoadError" surfaced by ct.1.8 tester (2026-05-11).
Pre-iter, non-JSON `ail check` (+8 sibling subcommands) propagated
loader errors via anyhow's Display, producing
`Error: <message>` without the bracketed `[code]` prefix the JSON
path emits.

Fix: new private helper `load_workspace_human(path)` in
crates/ail/src/main.rs that wraps ailang_surface::load_workspace,
threads any WorkspaceLoadError through workspace_error_to_diagnostic,
formats the resulting Diagnostic into a single stderr line
`error: [<code>] <def>: <message>`, and exits non-zero. Pure I/O
errors still propagate as anyhow errors.

Nine call sites swapped to the helper (check non-JSON arm, build,
run, emit-ir, prose, describe, deps, diff, manifest). The JSON arm
of `ail check` keeps its explicit `match` — different output
contract (structured stdout array).

The BareCrossModuleTypeRef Diagnostic message gained the existing
migration-command hint ("Run `ail migrate-canonical-types` to fix
legacy fixtures.") so the ct.1.8 actionable-hint test stays green.
The hint previously rode on WorkspaceLoadError's thiserror Display;
making workspace_error_to_diagnostic the single source for both
JSON and human paths required moving it into the Diagnostic body.

RED test crates/ail/tests/cli_diag_human_workspace_load_error.rs
pins the bracketed-code surface on a missing-import workspace
(tempdir-based fixture; no committed corpus addition).

cargo test --workspace: 565/0/3. clippy + doc: zero warnings.
Roadmap entry struck.

crates/ail/src/main.rs

@@ -336,7 +336,7 @@ fn main() -> Result<()> {
             if workspace {
                 // Workspace mode: load all modules and emit their defs
                 // together, alphabetically by (module, name).
-                let ws = ailang_surface::load_workspace(&path)?;
+                let ws = load_workspace_human(&path)?;
                 let mut entries: Vec<(String, &ailang_core::Def)> = Vec::new();
                 for (mod_name, m) in &ws.modules {
                     for d in &m.defs {
@@ -527,7 +527,7 @@ fn main() -> Result<()> {
         }
         Cmd::Describe { path, name, json, workspace } => {
             if workspace {
-                let ws = ailang_surface::load_workspace(&path)?;
+                let ws = load_workspace_human(&path)?;
                 let (mod_name, def) = resolve_describe_name(&ws, &name)?;
                 if json {
                     // We pass the def itself through and add the
@@ -608,7 +608,7 @@ fn main() -> Result<()> {
                     std::process::exit(1);
                 }
             } else {
-                let ws = ailang_surface::load_workspace(&path)?;
+                let ws = load_workspace_human(&path)?;
                 let diags = ailang_check::check_workspace(&ws);
                 if !diags.is_empty() {
                     for d in &diags {
@@ -648,7 +648,7 @@ fn main() -> Result<()> {
         Cmd::EmitIr { path, out } => {
             // Iter 5c: workspace lowering. For single-module programs the
             // workspace is effectively a trivial workspace with one module.
-            let ws = ailang_surface::load_workspace(&path)?;
+            let ws = load_workspace_human(&path)?;
             let diags = ailang_check::check_workspace(&ws);
             if !diags.is_empty() {
                 for d in &diags {
@@ -743,8 +743,8 @@ fn main() -> Result<()> {
         }
         Cmd::Diff { a, b, json, workspace } => {
             if workspace {
-                let ws_a = ailang_surface::load_workspace(&a)?;
-                let ws_b = ailang_surface::load_workspace(&b)?;
+                let ws_a = load_workspace_human(&a)?;
+                let ws_b = load_workspace_human(&b)?;
                 let report = build_workspace_diff(&ws_a, &ws_b);
                 if json {
                     let v = workspace_diff_report_to_json(&report);
@@ -771,7 +771,7 @@ fn main() -> Result<()> {
             }
         }
         Cmd::Workspace { entry, json } => {
-            let ws = ailang_surface::load_workspace(&entry)?;
+            let ws = load_workspace_human(&entry)?;
             // Iterate alphabetically over module names (BTreeMap order
             // is already sorted; assert it explicitly).
             let mut entries: Vec<(String, String, usize)> = ws
@@ -852,7 +852,7 @@ fn main() -> Result<()> {
         }
         Cmd::Deps { path, of, json, workspace } => {
             if workspace {
-                let ws = ailang_surface::load_workspace(&path)?;
+                let ws = load_workspace_human(&path)?;
 
                 // `--of NAME`: optional, accepts dot notation
                 // (`<module>.<def>`) or a bare name (matches in all
@@ -1237,7 +1237,8 @@ fn workspace_error_to_diagnostic(
                 "bare-cross-module-type-ref",
                 format!(
                     "module `{module}` contains bare type name `{name}` that does not resolve to a local type; \
-                    candidates from imports: {candidates:?}"
+                    candidates from imports: {candidates:?}. \
+                    Run `ail migrate-canonical-types` to fix legacy fixtures."
                 ),
             )
             .with_ctx(serde_json::json!({
@@ -1326,6 +1327,42 @@ fn workspace_error_to_diagnostic(
     }
 }
 
+/// Iter cli-diag-human (2026-05-14): loads a workspace and, on
+/// `WorkspaceLoadError`, routes through `workspace_error_to_diagnostic`
+/// to print a stderr line matching the JSON path's `[code]`-bracketed
+/// format before exiting non-zero. Pure I/O errors have no diagnostic
+/// equivalent (see `workspace_error_to_diagnostic` returning `None`
+/// for `W::Io`) and still propagate as anyhow errors so the caller's
+/// `?`-flow stays well-behaved.
+///
+/// Replaces the bare `ailang_surface::load_workspace(<path>)?` shape
+/// in every non-JSON CLI subcommand. The JSON path of `ail check`
+/// (which builds a structured diagnostics array on stdout) is the
+/// one site that keeps the explicit `match` against the loader
+/// result — it does not call this helper.
+fn load_workspace_human(
+    path: &Path,
+) -> Result<ailang_core::Workspace> {
+    match ailang_surface::load_workspace(path) {
+        Ok(ws) => Ok(ws),
+        Err(e) => match workspace_error_to_diagnostic(&e) {
+            Some(d) => {
+                eprintln!(
+                    "error: [{}] {}{}",
+                    d.code,
+                    d.def
+                        .as_ref()
+                        .map(|n| format!("{n}: "))
+                        .unwrap_or_default(),
+                    d.message,
+                );
+                std::process::exit(1);
+            }
+            None => Err(anyhow::anyhow!(e)),
+        },
+    }
+}
+
 /// Collects the *external* references of a definition: everything its body
 /// depends on that lives outside the def itself. Filtered out:
 ///
@@ -2197,7 +2234,7 @@ fn build_to(
     opt: &str,
     alloc: ailang_codegen::AllocStrategy,
 ) -> Result<PathBuf> {
-    let ws = ailang_surface::load_workspace(path)?;
+    let ws = load_workspace_human(path)?;
     let diags = ailang_check::check_workspace(&ws);
     if !diags.is_empty() {
         for d in &diags {

crates/ail/tests/cli_diag_human_workspace_load_error.rs

@@ -0,0 +1,73 @@
+//! Iter cli-diag-human (2026-05-14): non-JSON `ail check` and
+//! sibling subcommands surface `WorkspaceLoadError` with the same
+//! bracketed `[code]` prefix that the JSON path emits.
+//!
+//! Before this iter, non-JSON CLI subcommands that called
+//! `ailang_surface::load_workspace(&path)?` propagated the error
+//! through `anyhow`'s Display, producing `Error: <message>` —
+//! missing the bracketed diagnostic code that `--json` consumers
+//! and humans alike use to triage. Fix: a `load_workspace_human`
+//! helper routes through `workspace_error_to_diagnostic` and
+//! prints `error: [<code>] <def>: <message>` before exiting.
+//!
+//! These tests pin the bracketed-code surface on a representative
+//! workspace-load error (`module-not-found`). They are tempdir-
+//! based: the fixture is authored at test time and removed at
+//! drop, so no committed fixture is needed (the surface under
+//! test is the CLI formatting, not a permanent corpus entry).
+
+use std::io::Write;
+use std::path::PathBuf;
+use std::process::Command;
+
+fn ail_bin() -> PathBuf {
+    PathBuf::from(env!("CARGO_BIN_EXE_ail"))
+}
+
+/// Build a fresh tempdir containing one .ail module that imports a
+/// non-existent sibling module. Returns the entry path. The tempdir
+/// is leaked deliberately — tests are single-shot and tempdir
+/// cleanup is the test harness's concern.
+fn tempdir_with_missing_import() -> PathBuf {
+    let dir = std::env::temp_dir().join(format!(
+        "ail_cli_diag_human_{}",
+        std::process::id()
+    ));
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(&dir).expect("create tempdir");
+    let entry = dir.join("test_missing_import.ail");
+    let mut f = std::fs::File::create(&entry).expect("create entry .ail");
+    writeln!(
+        f,
+        "(module test_missing_import\n  (import this_module_does_not_exist)\n  (fn main\n    (type (fn-type (params) (ret (con Unit)) (effects IO)))\n    (params)\n    (body (do io/print_str \"hello\"))))"
+    ).expect("write fixture");
+    entry
+}
+
+/// Property: `ail check <fixture>` (non-JSON path) on a workspace
+/// with a missing import emits a stderr line of shape
+/// `error: [module-not-found] ...` and exits non-zero. Pre-iter
+/// behaviour was `Error: module ...` without the bracketed code;
+/// this is the regression guard against re-introducing that.
+#[test]
+fn check_non_json_emits_bracketed_module_not_found() {
+    let fixture = tempdir_with_missing_import();
+    let output = Command::new(ail_bin())
+        .args(["check", fixture.to_str().unwrap()])
+        .output()
+        .expect("ail binary must launch");
+    assert!(
+        !output.status.success(),
+        "ail check must exit non-zero on missing import; stderr={}",
+        String::from_utf8_lossy(&output.stderr),
+    );
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("[module-not-found]"),
+        "expected `[module-not-found]` in stderr, got: {stderr}"
+    );
+    assert!(
+        stderr.starts_with("error: "),
+        "expected stderr to start with `error: ` (lowercase, matching the JSON-mode wording), got: {stderr}"
+    );
+}