The round-2 fix acquired _available_models_cache_lock in the resolver to read
the snapshot+fingerprint consistently. Codex found that introduces a DEADLOCK:
config-save takes _cfg_lock -> _available_models_cache_lock while catalog-refresh
takes _available_models_cache_lock -> _cfg_lock (lock-order inversion), and it
also makes every send wait behind a catalog rebuild.
Fix: publish an immutable (snapshot, publisher_fingerprint) tuple atomically via
_sync_models_cache_provenance() at every cache publish/invalidate site (9 sites),
and have _endpoint_advertised_model_ids read that single global with ONE lock-free
load. Reading one tuple can't tear (GIL), acquires no lock (no ordering edge, no
deadlock), and never blocks behind a rebuild. A reader racing a publish sees the
previous consistent pair, never a torn snapshot/fingerprint.
Verified: two-thread probe — resolver completes in 0.0001s while another thread
holds _available_models_cache_lock (was the deadlock). Real catalog build publishes
the provenance tuple and closes the P0 end-to-end. Adds a deadlock-regression test.
1443288dee