Add rescore index for ANN queries

Add rescore index type: stores full-precision float vectors in a rowid-keyed
shadow table, quantizes to int8 for fast initial scan, then rescores top
candidates with original vectors. Includes config parser, shadow table
management, insert/delete support, KNN integration, compile flag
(SQLITE_VEC_ENABLE_RESCORE), fuzz targets, and tests.

benchmarks-ann/Makefile

@@ -21,9 +21,14 @@ BASELINES = \
 # ANNOY_CONFIGS = \
 # 	"annoy-t50:type=annoy,n_trees=50"
 
-ALL_CONFIGS = $(BASELINES)
+RESCORE_CONFIGS = \
+	"rescore-bit-os8:type=rescore,quantizer=bit,oversample=8" \
+	"rescore-bit-os16:type=rescore,quantizer=bit,oversample=16" \
+	"rescore-int8-os8:type=rescore,quantizer=int8,oversample=8"
 
-.PHONY: seed ground-truth bench-smoke bench-10k bench-50k bench-100k bench-all \
+ALL_CONFIGS = $(BASELINES) $(RESCORE_CONFIGS)
+
+.PHONY: seed ground-truth bench-smoke bench-rescore bench-10k bench-50k bench-100k bench-all \
         report clean
 
 # --- Data preparation ---
@@ -40,6 +45,10 @@ bench-smoke: seed
 	$(BENCH) --subset-size 5000 -k 10 -n 20 -o runs/smoke \
 		$(BASELINES)
 
+bench-rescore: seed
+	$(BENCH) --subset-size 10000 -k 10 -o runs/rescore \
+		$(RESCORE_CONFIGS)
+
 # --- Standard sizes ---
 bench-10k: seed
 	$(BENCH) --subset-size 10000 -k 10 -o runs/10k $(ALL_CONFIGS)

benchmarks-ann/bench.py

@@ -140,6 +140,39 @@ INDEX_REGISTRY["baseline"] = {
 }
 
 
+# ============================================================================
+# Rescore implementation
+# ============================================================================
+
+
+def _rescore_create_table_sql(params):
+    quantizer = params.get("quantizer", "bit")
+    oversample = params.get("oversample", 8)
+    return (
+        f"CREATE VIRTUAL TABLE vec_items USING vec0("
+        f"  chunk_size=256,"
+        f"  id integer primary key,"
+        f"  embedding float[768] distance_metric=cosine"
+        f"  indexed by rescore(quantizer={quantizer}, oversample={oversample}))"
+    )
+
+
+def _rescore_describe(params):
+    q = params.get("quantizer", "bit")
+    os = params.get("oversample", 8)
+    return f"rescore  {q} (os={os})"
+
+
+INDEX_REGISTRY["rescore"] = {
+    "defaults": {"quantizer": "bit", "oversample": 8},
+    "create_table_sql": _rescore_create_table_sql,
+    "insert_sql": None,
+    "post_insert_hook": None,
+    "run_query": None,  # default MATCH query works — rescore is automatic
+    "describe": _rescore_describe,
+}
+
+
 # ============================================================================
 # Config parsing
 # ============================================================================