Promotes the two throwaway sanitizer harnesses used to diagnose the
2026-08-16 soul crash into engram/test/ so the bug cannot silently regress.
The harness has two halves and the PAIR is the point — it is what localises
the defect to concurrency rather than to HNSW logic:
single 3000 clustered vectors, one thread, ASan+UBSan. The CONTROL.
Must always be clean. During diagnosis this cleared all 13,820
real dim-768 vectors from the live store, which DISPROVED an
inspection-derived hypothesis about an out-of-bounds
reverse-link write at engram_vindex.c:340.
concurrent writer + reader on one shared index, TSan. Currently reports a
race at engram_vindex.c:195 (visited_reset) reached from both
vindex_search and vindex_insert, because VIndex still owns its
visited[]/visit_epoch scratch — so even two concurrent READS
corrupt each other's traversal.
Verified: half 1 passes, half 2 reproduces the race.
Gated on EXPECT_RACE, default 1, so the concurrent half documents the known
defect without failing the suite today. When the visited set moves to a
per-query checkout pool (hnswlib VisitedListPool style — NOT thread_local,
since http_worker is a thread per connection and a __thread buffer would leak
~55KB per connection), flip EXPECT_RACE=0 and it becomes a real gate.
bigmerge
e99a4640e2