refactor(8.0): rename HnswProvider → VectorIndexProvider (8.0 scaffold)

Brainy 8.0 collapses the vector-index contract to algorithm-neutral names.
"Vector index" describes the role; "HNSW" was the name of one specific
implementation. The role name is what the public contract should carry;
the algorithm name belongs to the concrete class.

This is step 1 of the brainy 8.0 rename scaffolding tracked in handoff
thread BRAINY-8.0-RENAME-COORDINATION § A.1 (cortex shipped the matching
VectorIndexProvider in commit 0e4d637).

CHANGES

src/plugin.ts
- Primary interface name flipped: HnswProvider → VectorIndexProvider.
  Same byte-for-byte shape (8 methods, no signatures changed).
- HnswProvider kept as a deprecated type alias so call sites compile
  mid-migration. Removed in a later 8.0 commit.
- DiskAnnProvider was already a type alias of HnswProvider; redeclared
  as alias of VectorIndexProvider with a deprecation note explaining
  the 'diskann' provider key folds into 'vector' in 8.0.
- JSDoc updated to describe the implementation-neutral surface:
  Brainy's own JS HNSW + any native acceleration provider both
  satisfy it.

src/hnsw/hnswIndex.ts
- Internal class HNSWIndex now declares `implements VectorIndexProvider`
  instead of `implements HnswProvider`. Import + class comment updated.

NO-OP scope

No behavioural change. Every existing call site continues to work
because HnswProvider remains as a temporary alias. Tests + build green.

VERIFICATION

- npx tsc --noEmit: clean
- npm test: 1468 / 1468 unit
- npm run build: clean (verified at parent commit 89e4d81; this commit
  is type-only so doesn't change dist)

NEXT IN SCAFFOLDING

- Add 'vector' provider key registration alongside 'hnsw' (cache + cortex)
- Add config.vector top-level path alongside config.hnsw with recall preset
- Migrate brainy.ts call sites to the new names
- Rename HNSWIndex class → JsHnswVectorIndex (per planning § 2.3)
- Final cleanup commit: remove HnswProvider alias + config.hnsw + 'hnsw' key

src/plugin.ts

@@ -192,15 +192,20 @@ export interface GraphIndexProvider {
 }
 
 /**
- * The object returned by the `'hnsw'` provider factory — a drop-in for
- * `HNSWIndex`. Brainy calls this surface via `this.index.*` plus the
- * transactional add/remove operations. `enableCOW`, `getItem`, and
- * `setPersistMode` are intentionally absent: Brainy guards each with
- * feature-detection (`typeof x === 'function'`), so they are optional and not
- * part of the required contract (Brainy's own `HNSWIndex` omits
- * `setPersistMode`, for instance).
+ * The object returned by the `'vector'` provider factory — Brainy's vector
+ * index contract. Implementations include Brainy's own JS HNSW index and any
+ * native acceleration provider (e.g. cortex's Adaptive DiskANN).
+ *
+ * Brainy calls this surface via `this.index.*` plus the transactional add/remove
+ * operations. `enableCOW`, `getItem`, and `setPersistMode` are intentionally
+ * absent: Brainy guards each with feature-detection (`typeof x === 'function'`),
+ * so they are optional and not part of the required contract (Brainy's own JS
+ * HNSW index omits `setPersistMode`, for instance).
+ *
+ * **Provider key:** registered under `'vector'`. The legacy `'hnsw'` key is
+ * accepted as a compat shim in 8.0 and retired in a future release.
  */
-export interface HnswProvider {
+export interface VectorIndexProvider {
   addItem(item: VectorDocument): Promise<string>
   removeItem(id: string): Promise<boolean>
   search(
@@ -217,29 +222,22 @@ export interface HnswProvider {
 }
 
 /**
- * The `'diskann'` provider — a billion-scale alternative to HNSW backed
- * by cortex's pure-Rust Vamana + Product Quantization implementation
- * (see ADR-002 in the cortex repo).
- *
- * Structurally a drop-in for [`HnswProvider`]: brainy calls the same
- * `addItem` / `search` / `rebuild` surface and never needs to know
- * which index is underneath. The differences are operational:
- *
- * - **Memory footprint**: ~16 GB RAM at 1 B vectors (PQ codes only).
- *   HNSW would need ~1.5 TB to keep the full vectors resident.
- * - **Build model**: build-once, query-many. Dynamic insertions buffer
- *   to an in-memory delta brute-forced alongside the main index;
- *   `rebuild()` folds them in.
- * - **Storage**: requires a local filesystem path (NVMe SSD for the
- *   published latency numbers). Cloud-storage adapters continue using
- *   HNSW.
- *
- * Engagement is automatic when the cortex provider is registered, the
- * storage adapter exposes `getBinaryBlobPath`, and the metadata index
- * has a stable `idMapper`. Users force the legacy index via
- * `config.index.type = 'hnsw'`.
+ * @deprecated Renamed to {@link VectorIndexProvider} in Brainy 8.0. The
+ * interface shape is byte-for-byte identical; only the name changed because
+ * "vector index" describes the role, while "HNSW" was the name of one
+ * specific implementation. This alias is provided as a compatibility shim
+ * for code mid-migration; new code should import `VectorIndexProvider`.
  */
-export type DiskAnnProvider = HnswProvider
+export type HnswProvider = VectorIndexProvider
+
+/**
+ * @deprecated The `'diskann'` provider key has been folded into `'vector'` in
+ * Brainy 8.0. The vector index contract is implementation-neutral: any provider
+ * registered under `'vector'` may be HNSW, DiskANN, or a future algorithm. The
+ * type alias is preserved so existing imports compile while consumers migrate
+ * to `VectorIndexProvider`.
+ */
+export type DiskAnnProvider = VectorIndexProvider
 
 /**
  * The `'entityIdMapper'` provider — a drop-in for `EntityIdMapper`. Injected