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>

benchmarks-ann/bench-delete/Makefile

@@ -0,0 +1,41 @@
+BENCH = python bench_delete.py
+EXT = ../../dist/vec0
+
+# --- Configs to test ---
+FLAT = "flat:type=vec0-flat,variant=float"
+RESCORE_BIT = "rescore-bit:type=rescore,quantizer=bit,oversample=8"
+RESCORE_INT8 = "rescore-int8:type=rescore,quantizer=int8,oversample=8"
+DISKANN_R48 = "diskann-R48:type=diskann,R=48,L=128,quantizer=binary"
+DISKANN_R72 = "diskann-R72:type=diskann,R=72,L=128,quantizer=binary"
+
+ALL_CONFIGS = $(FLAT) $(RESCORE_BIT) $(RESCORE_INT8) $(DISKANN_R48) $(DISKANN_R72)
+
+DELETE_PCTS = 5,10,25,50,75,90
+
+.PHONY: smoke bench-10k bench-50k bench-all report clean
+
+# Quick smoke test (small dataset, few queries)
+smoke:
+	$(BENCH) --subset-size 5000 --delete-pct 10,50 -k 10 -n 20 \
+		--dataset cohere1m --ext $(EXT) \
+		$(FLAT) $(DISKANN_R48)
+
+# Standard benchmarks
+bench-10k:
+	$(BENCH) --subset-size 10000 --delete-pct $(DELETE_PCTS) -k 10 -n 50 \
+		--dataset cohere1m --ext $(EXT) $(ALL_CONFIGS)
+
+bench-50k:
+	$(BENCH) --subset-size 50000 --delete-pct $(DELETE_PCTS) -k 10 -n 50 \
+		--dataset cohere1m --ext $(EXT) $(ALL_CONFIGS)
+
+bench-all: bench-10k bench-50k
+
+# Query saved results
+report:
+	@echo "Query results:"
+	@echo "  sqlite3 runs/cohere1m/10000/delete_results.db \\"
+	@echo "    \"SELECT config_name, delete_pct, recall, query_mean_ms, vacuum_size_mb FROM delete_runs ORDER BY config_name, delete_pct\""
+
+clean:
+	rm -rf runs/