fix(test): anchor all test sandboxes on reclaimable pid-free names

Every hand-rolled test-sandbox helper keyed its directory on
std::process::id() under env::temp_dir(), so the pre-create wipe never
matched a prior run's path: one directory stranded per helper site per
test-binary invocation, >50k dirs / 15.5 GiB on the 16 GiB tmpfs by
2026-07-13.

The remedy extends commit 84e1075's knob-lab pattern to every site:
fixed, tag-keyed, PID-FREE names under the build-tree tmp anchor, each
site keeping its pre-create remove_dir_all — which now genuinely
reclaims the previous run. Integration/bench targets use
env!("CARGO_TARGET_TMPDIR"); src-internal unit-test helpers (cargo sets
that var only for integration/bench targets) derive the same anchor
from env!("CARGO_MANIFEST_DIR") + ../../target/tmp. Residue is bounded
to one directory per site, on disk instead of the tmpfs, per checkout.

One deliberate exception: project_new.rs's tmp() stays under
env::temp_dir() because two of its tests (new_outside_a_work_tree_*)
need a base genuinely detached from any git work tree, and target/
lives inside the checkout's work tree. Its name carries a per-checkout
hash discriminator instead, so concurrently running checkouts (which
share /tmp) cannot race on the fixed name while each run still
reclaims its predecessor.

tempfile-RAII was considered and rejected (issue #258 decision log):
it cannot cover the process-lifetime OnceLock scaffolds (statics never
drop), adds a dependency, and still strands kill-leftovers on the
tmpfs.

Verified: full workspace suite green (incl. the new RED regression
test temp_cwd_sandbox_does_not_leak_under_env_temp_dir), clippy clean,
sandboxes observed under target/tmp with stable names across runs.
Stale pre-fix PID-keyed dirs in /tmp are not swept by this change and
age out only by manual cleanup.

closes #258

crates/aura-registry/src/lib.rs

@@ -909,8 +909,11 @@ mod tests {
     }
 
     fn temp_path(name: &str) -> PathBuf {
-        // unique per test + per process; no external tempfile dependency
-        std::env::temp_dir().join(format!("aura-registry-{}-{}.jsonl", std::process::id(), name))
+        // fixed, tag-keyed, pid-free name under the build-tree tmp anchor (#258):
+        // no external tempfile dependency, and every call site already wipes
+        // before use, so the fixed name genuinely reclaims the previous run.
+        std::path::Path::new(concat!(env!("CARGO_MANIFEST_DIR"), "/../../target/tmp"))
+            .join(format!("aura-registry-{name}.jsonl"))
     }
 
     #[test]
@@ -1081,8 +1084,10 @@ mod tests {
     fn temp_family_dir(name: &str) -> PathBuf {
         // a unique per-test directory so the families.jsonl sibling never collides
         // across tests sharing the temp dir; the registry binds to runs.jsonl inside.
-        let dir =
-            std::env::temp_dir().join(format!("aura-registry-fam-{}-{}", std::process::id(), name));
+        // Fixed, tag-keyed, pid-free name under the build-tree tmp anchor (#258): the
+        // pre-create wipe below then genuinely reclaims the previous run.
+        let dir = std::path::Path::new(concat!(env!("CARGO_MANIFEST_DIR"), "/../../target/tmp"))
+            .join(format!("aura-registry-fam-{name}"));
         let _ = fs::remove_dir_all(&dir);
         fs::create_dir_all(&dir).expect("create temp family dir");
         dir.join("runs.jsonl")

crates/aura-registry/src/trace_store.rs

@@ -288,8 +288,10 @@ mod tests {
     use aura_core::{Scalar, ScalarKind, Timestamp};
 
     fn temp_traces_root(name: &str) -> PathBuf {
-        let dir =
-            std::env::temp_dir().join(format!("aura-tracestore-{}-{}", std::process::id(), name));
+        // fixed, tag-keyed, pid-free name under the build-tree tmp anchor (#258): the
+        // pre-create wipe below then genuinely reclaims the previous run.
+        let dir = std::path::Path::new(concat!(env!("CARGO_MANIFEST_DIR"), "/../../target/tmp"))
+            .join(format!("aura-tracestore-{name}"));
         let _ = fs::remove_dir_all(&dir);
         fs::create_dir_all(&dir).expect("create temp traces root");
         dir

crates/aura-registry/tests/campaign_run_store_e2e.rs

@@ -18,8 +18,8 @@ use aura_registry::{
 };
 
 fn temp_dir(name: &str) -> PathBuf {
-    let dir = std::env::temp_dir()
-        .join(format!("aura-registry-campaign-e2e-{}-{}", std::process::id(), name));
+    let dir = std::path::Path::new(env!("CARGO_TARGET_TMPDIR"))
+        .join(format!("aura-registry-campaign-e2e-{name}"));
     let _ = fs::remove_dir_all(&dir);
     fs::create_dir_all(&dir).expect("create temp dir");
     dir.join("runs.jsonl")