refactor(8.0): rename HNSWIndex class → JsHnswVectorIndex (scaffold step 3)

Step 3 of the brainy 8.0 rename scaffolding. The class name now matches
its role: the JS HNSW implementation of the VectorIndexProvider contract.
Per planning § 2.3: this is NOT cosmetic — leaving HNSWIndex when the
public contract is VectorIndexProvider creates a confusing read at the
implementation layer.

CHANGES

Repo-wide mechanical rename: HNSWIndex → JsHnswVectorIndex across 52
references in src/ + 5 references in tests/ via sed. Class declaration,
imports, JSDoc, comments, and identifiers all updated.

src/index.ts
- Primary public export: JsHnswVectorIndex (the new name).
- Backwards-compat alias: `export const HNSWIndex = JsHnswVectorIndex`
  with @deprecated note. Removed in the final 8.0 cleanup commit.

NO-OP scope

No behavioural change. The class is exactly the same; only the name
moved. Tests + build green.

VERIFICATION

- npx tsc --noEmit: clean
- npm test: 1468 / 1468 unit

src/index.ts

@@ -296,10 +296,20 @@ import type {
   StorageAdapter
 } from './coreTypes.js'
 
-// Export HNSW index
-import { HNSWIndex } from './hnsw/hnswIndex.js'
+// Export vector index implementation (the JS HNSW path)
+import { JsHnswVectorIndex } from './hnsw/hnswIndex.js'
 
-export { HNSWIndex }
+export { JsHnswVectorIndex }
+
+/**
+ * @deprecated Renamed to {@link JsHnswVectorIndex} in Brainy 8.0. The
+ * implementation is unchanged — the new name disambiguates the JS HNSW
+ * path from any native vector-index provider (DiskANN, etc.) that may be
+ * loaded via the `'vector'` provider key. This alias is provided as a
+ * compatibility shim for code mid-migration and is removed in the 8.0
+ * final cleanup commit.
+ */
+export const HNSWIndex = JsHnswVectorIndex
 
 export type {
   Vector,