Add text PK, WAL concurrency tests, and fix bench-smoke config

Infrastructure improvements:
- Fix benchmarks-ann Makefile: type=baseline -> type=vec0-flat (baseline
  was never a valid INDEX_REGISTRY key)
- Add DiskANN + text primary key test: insert, KNN, delete, KNN
- Add rescore + text primary key test: insert, KNN, delete, KNN
- Add WAL concurrency test: reader sees snapshot isolation while
  writer has an open transaction, KNN works on reader's snapshot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>