Filter deleted nodes from DiskANN search results and add delete tests

DiskANN's delete repair only fixes forward edges (nodes the deleted
node pointed to). Stale reverse edges can cause deleted rowids to
appear in search results. Fix: track a 'confirmed' flag on each
search candidate, set when the full-precision vector is successfully
read during re-ranking. Only confirmed candidates are included in
output. Zero additional SQL queries — piggybacks on the existing
re-rank vector read.

Also adds delete hardening tests:
- Rescore: interleaved delete+KNN, rowid_in after deletes, full
  delete+reinsert cycle
- DiskANN: delete+reinsert cycles with KNN verification, interleaved
  delete+KNN

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