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/utils/periodicCleanup.ts

@@ -11,7 +11,7 @@
 import { prodLog } from './logger.js'
 import { DELETED_FIELD, isDeleted } from './metadataNamespace.js'
 import type { StorageAdapter } from '../coreTypes.js'
-import type { HNSWIndex } from '../hnsw/hnswIndex.js'
+import type { JsHnswVectorIndex } from '../hnsw/hnswIndex.js'
 import type { MetadataIndexManager } from './metadataIndex.js'
 
 export interface CleanupConfig {
@@ -44,7 +44,7 @@ export interface CleanupStats {
  */
 export class PeriodicCleanup {
   private storage: StorageAdapter
-  private hnswIndex: HNSWIndex
+  private hnswIndex: JsHnswVectorIndex
   private metadataIndex: MetadataIndexManager | null
   private config: CleanupConfig
   private stats: CleanupStats
@@ -53,7 +53,7 @@ export class PeriodicCleanup {
 
   constructor(
     storage: StorageAdapter,
-    hnswIndex: HNSWIndex,
+    hnswIndex: JsHnswVectorIndex,
     metadataIndex: MetadataIndexManager | null,
     config: Partial<CleanupConfig> = {}
   ) {