HNSW vector index: 100K search in 27ms (58x faster than brute-force)

- instant-distance HNSW implementation for approximate nearest neighbors
- HnswStore: build from stored embeddings, in-memory index, thread-safe
- POST /vectors/hnsw/build — build index from Parquet (100K in 35s release)
- POST /vectors/hnsw/search — fast ANN search
- GET /vectors/hnsw/list — list loaded indexes

Benchmark (100K × 768d, release build):
  Brute-force: 1,567ms
  HNSW:           31ms (50x)
  HNSW warm:      27ms (58x)

Build cost: 35s one-time for 100K vectors (release mode)
ef_construction=40, ef_search=50 — good recall/speed balance

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

Cargo.lock

@@ -2596,6 +2596,12 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hermit-abi"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc0fef456e4baa96da950455cd02c081ca953b141298e41db3fc7e36b1da849c"
+
 [[package]]
 name = "hex"
 version = "0.4.3"
@@ -2921,6 +2927,19 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "instant-distance"
+version = "0.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8c619cdaa30bb84088963968bee12a45ea5fbbf355f2c021bcd15589f5ca494a"
+dependencies = [
+ "num_cpus",
+ "ordered-float 3.9.2",
+ "parking_lot",
+ "rand 0.8.5",
+ "rayon",
+]
+
 [[package]]
 name = "integer-encoding"
 version = "3.0.4"
@@ -3550,6 +3569,16 @@ dependencies = [
  "libm",
 ]
 
+[[package]]
+name = "num_cpus"
+version = "1.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91df4bbde75afed763b708b7eee1e8e7651e02d97f6d5dd763e89367e957b23b"
+dependencies = [
+ "hermit-abi",
+ "libc",
+]
+
 [[package]]
 name = "num_enum"
 version = "0.7.6"
@@ -3688,6 +3717,15 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "ordered-float"
+version = "3.9.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1e1c390732d15f1d48471625cd92d154e66db2c56645e29a9cd26f4699f72dc"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "parking_lot"
 version = "0.12.5"
@@ -4906,7 +4944,7 @@ checksum = "7e54bc85fc7faa8bc175c4bab5b92ba8d9a3ce893d0e9f42cc455c8ab16a9e09"
 dependencies = [
  "byteorder",
  "integer-encoding",
- "ordered-float",
+ "ordered-float 2.10.1",
 ]
 
 [[package]]
@@ -5439,6 +5477,7 @@ dependencies = [
  "axum",
  "bytes",
  "chrono",
+ "instant-distance",
  "object_store",
  "parquet",
  "serde",