A returned write must be immediately readable by id, metadata filter, vector
search, and graph traversal — no await, delay, or retry between the write
returning and the read. This holds by construction because every projection
(canonical + HNSW + metadata index + graph index) commits inside the write's
transaction; the generation counter is the {seq} a caller can pin. The test
locks that guarantee so index maintenance can never quietly move off the commit
path (which would turn a returned write into a not-yet-queryable one).
First brainy chapter of the write/index-spine hardening program.
