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-ingest/src/lib.rs

@@ -551,6 +551,12 @@ mod tests {
     /// mirrors `aura-cli`'s own `tests/common::mint_tempdir` pattern (unique
     /// name via pid + an atomic counter) so this crate's tests need no
     /// `tempfile` dev-dependency for a handful of throwaway fixture dirs.
+    ///
+    /// #258 sweep note: deliberately kept pid-suffixed like the sibling it
+    /// mirrors — there is no pre-create wipe here for a stale pid to defeat;
+    /// `Drop` (below) unconditionally reclaims the directory when the owning
+    /// test ends, so nothing strands across runs. The pid instead
+    /// disambiguates concurrent `cargo test` processes.
     struct ScratchDir(std::path::PathBuf);
 
     impl ScratchDir {