Records which (repo, git-ref) combinations produced each cache
fingerprint. Later slices will wire this to a nightly Gitea sweep
that queries /api/v1/repos/.../branches and /tags, then evicts
fingerprints whose recorded refs are all gone AND whose
last_seen_unix is older than the retention window.
Per-fingerprint (not per-blob) because:
* Fingerprints are the cache keys claw-cargo uses. Tracking at the
fp layer keeps this aligned with the claw-cargo boundary.
* Blobs are content-addressed and may be shared. Ref-tracking is
about "why we kept this cache" — a per-fp concern.
New module cluster::ref_tracking:
* RefEntry { fingerprint, repo, refs, first_seen_unix, last_seen_unix }
* RefTracking::record(fp, repo, git_ref, now) — creates or updates
* RefTracking::get(fp) / list_all() / forget(fp)
* RefTracking::stale_at(now, live_refs_by_repo, retention_secs) →
Vec<fingerprint>, the deletion-eligibility list
On-disk: <root>/ref-tracking/<hh>/<fp_hex>.json. JSON so operators
can inspect with jq. One record per cached fp; even 100k fps is
under 50 MB.
Semantics baked in:
* record() APPENDS refs, never removes — sweep decides staleness
* record() rejects repo change for a fp (collision or bug detector)
* refs stable-sorted in-file so cross-node diff is easy
* stale_at treats "repo not in live_refs map" as "all refs dead"
→ deleted repos don't leak caches
* retention_secs is a floor: dead-but-fresh caches survive
+10 tests: create, append-and-refresh, dedup, repo-change reject,
stale-at happy path, stale-at missing-repo, stale-at retention,
forget truth values, list sorted, validate rejects.
365 tests pass (+10). Pre-existing macOS
hot::tests::test_project_target_size_bytes failure unchanged.
No CLI or wire integration in this PR — pure library, testable
in isolation. Follow-ons: (a) claw-cargo auto-record on cache put,
(b) Gitea polling adapter, (c) sweep wired into cluster-gc.
