Found by running the standalone server end to end: after a clean shutdown that
logged "persisted HNSW graphs to disk (next boot loads instead of rebuilding)",
the next boot logged
WARN persisted HNSW graph failed to load; falling back to rebuild
slot="content_vector" error=USearch load failed: Failed to read vectors
and rebuilt the index. Every boot. Correct results, no data loss, a WARN nobody
reads - and the optimization was dead.
Cause: `checkpoint_graphs` saves whichever index the slot holds, and
`build_slot_index` holds a BRUTE-FORCE index below the dimension-aware
crossover. Both write to the same `<kind>__<slot>.usearch` path, so a small slot
persisted a `BFVI` file that `load_persisted_slot` then handed to the USearch
reader. Observed on a 6-item, 128-D store; at production scale the slot is
USearch-backed, which is why the cluster never surfaced it - but every dev,
staging, and small-tenant instance pays a full index rebuild on every start, and
that rebuild is what the 20-minute startup probe budget exists for.
Fix: sniff the file's magic (brute-force `MAGIC` is now `pub(crate)` so the
reader and writer cannot drift) and dispatch to the matching loader. A
brute-force graph is still rejected when `expected_count` has grown past the
crossover, so the rebuild upgrades the backend to HNSW rather than pinning a
linear scan forever. Log lines now say "vector graph" and name the backend
instead of claiming HNSW for both.
Tests: `brute_force_slot_graph_round_trips_through_registry_persistence` covers
the case that was broken (the existing round-trip test forces USearch, which is
why it passed throughout), and
`grown_corpus_rejects_a_brute_force_graph_so_the_rebuild_upgrades_it` pins the
upgrade path. Verified live: the same data directory that produced the WARN now
logs `loaded persisted vector graph from disk (skipped full rebuild)
backend="brute-force" count=6`, with feed, text search, and vector search all
returning the pre-restart results.
e5fd19eb73