Add delete recall benchmark suite

New benchmarks-ann/bench-delete/ directory measures KNN recall
degradation after random row deletion across index types (flat,
rescore, IVF, DiskANN). For each config and delete percentage:
builds index, measures baseline recall, copies DB, deletes random
rows, measures post-delete recall, VACUUMs and records size savings.

Includes Makefile targets, self-contained smoke test with synthetic
data, and results DB for analysis.

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