refactor(8.0): final cleanup — drop HnswProvider alias + config.hnsw + 'hnsw' surface (scaffold step 6)

Step 6 of the brainy 8.0 rename scaffolding. Removes every legacy 'hnsw'-
flavoured public name introduced as a compat shim in steps 1-5. The
algorithm-neutral surface is now the only surface.

REMOVED — PHASE A (type aliases)

src/plugin.ts
- Removed `export type HnswProvider = VectorIndexProvider` alias.
- Removed `export type DiskAnnProvider = VectorIndexProvider` alias.

src/index.ts
- Removed `export const HNSWIndex = JsHnswVectorIndex` alias.

src/types/brainy.types.ts
- Removed `BrainyStats.indexHealth.hnsw` field (the new `vector` field
  carries the same boolean).

src/brainy.ts
- `stats()` no longer emits the legacy `hnsw` field on `indexHealth`.

REMOVED — PHASE B (storage adapter rename, 8 adapters)

Repo-wide rename: `saveHNSWData` → `saveVectorIndexData` and
`getHNSWData` → `getVectorIndexData` across:

- src/storage/adapters/baseStorageAdapter.ts (abstract declarations)
- src/storage/adapters/fileSystemStorage.ts
- src/storage/adapters/gcsStorage.ts
- src/storage/adapters/r2Storage.ts
- src/storage/adapters/s3CompatibleStorage.ts
- src/storage/adapters/azureBlobStorage.ts
- src/storage/adapters/opfsStorage.ts
- src/storage/adapters/memoryStorage.ts
- src/storage/adapters/historicalStorageAdapter.ts
- src/brainy.ts (call sites)
- src/hnsw/hnswIndex.ts + src/hnsw/typeAwareHNSWIndex.ts (call sites)

The default-delegation wrappers added in scaffold step 4 are removed
(would have been duplicate declarations after the rename).

REMOVED — PHASE C (cache category 'hnsw')

src/utils/unifiedCache.ts
- Cache-category union narrowed from
    'hnsw' | 'vectors' | 'metadata' | 'embedding' | 'other'
  to
    'vectors' | 'metadata' | 'embedding' | 'other'
- typeAccessCounts, typeSizes, typeCounts, accessRatios, sizeRatios,
  and the fairness-check iterator all drop the 'hnsw' key.
- Per planning § 2.5: pre-8.0 'hnsw' cache entries are an in-memory
  category. Cache is rebuildable; entries naturally don't exist after
  a restart, so no migration path is needed.

src/hnsw/hnswIndex.ts
- 3 cache.set() call sites migrated from category 'hnsw' to 'vectors'.
- Cache key prefix `hnsw:vector:` → `vector:`.
- typeCounts/typeSizes/typeAccessCounts accessor renames
  `.hnsw` → `.vectors`.

tests/unit/utils/unifiedCache-eviction.test.ts
- Test fixtures updated: cache.set(..., 'hnsw', ...) → cache.set(..., 'vectors', ...).
- Stats assertion `stats.typeSizes.hnsw` → `stats.typeSizes.vectors`.

REMOVED — PHASE D (config.hnsw)

src/types/brainy.types.ts
- Removed `BrainyConfig.hnsw` field entirely. The 8.0 surface is
  `BrainyConfig.vector.{recall, quantization, vectorStorage, advanced}`.

src/brainy.ts
- normalizeConfig() no longer emits a `hnsw` field on Required<BrainyConfig>.
- setupIndex() rewired:
  - Reads from `this.config.vector` (not `this.config.hnsw`).
  - Calls `resolveJsHnswConfig(this.config.vector)` to translate the
    `recall` preset into M / efConstruction / efSearch knobs (with
    `advanced.hnsw` overrides winning when supplied).
- Imports `resolveJsHnswConfig` from './utils/recallPreset.js'.

NOT IN THIS COMMIT (deliberately)

- Persisted file path migration `_system/hnsw-*.json` →
  `_system/vector-index-*.json`. Requires dual-read logic on boot
  across 8 storage adapters. Per integration doc lines 531-539:
  "Reader accepts either spelling on load; writer emits the new spelling
  only; brains self-migrate on the next persist after upgrade." This is
  a separate body of work and lands in a follow-up commit before 8.0 GA.
- `config.hnswPersistMode` top-level field is unchanged. It's not in
  the rename inventory; a future commit can fold it into
  `config.vector.persistMode` if desired.
- strictConfig enforcement wiring. The field is accepted by
  normalizeConfig() with default 'warn'; the actual warning emission at
  knob-mismatch sites lands when the config-resolution layer is touched
  for the persisted-path migration.

VERIFICATION

- npx tsc --noEmit: clean
- npm test: 1468 / 1468 unit (one test fixture updated to use the new
  category name; assertion still passes after fixture rename)
- npm run build: clean

The 8.0 PR's user-facing surface is now algorithm-neutral end-to-end:
VectorIndexProvider, config.vector.recall, JsHnswVectorIndex,
saveVectorIndexData, 'vectors' cache category, indexHealth.vector,
strictConfig — all standalone. No legacy 'hnsw' name remains in any
public type or method signature.

tests/unit/utils/unifiedCache-eviction.test.ts

@@ -20,7 +20,7 @@ describe('UnifiedCache Eviction Scoring', () => {
     cache.set('meta1', { data: 'metadata' }, 'metadata', 100, 1)
 
     // Add HNSW vector (high rebuild cost, high access count)
-    cache.set('hnsw1', { data: 'vector' }, 'hnsw', 100, 50)
+    cache.set('hnsw1', { data: 'vector' }, 'vectors', 100, 50)
 
     // Simulate frequent access to HNSW
     for (let i = 0; i < 10; i++) {
@@ -41,7 +41,7 @@ describe('UnifiedCache Eviction Scoring', () => {
 
     // Metadata should likely be evicted (low value)
     // Note: Since we're filling most of cache, metadata is candidate for eviction
-    expect(stats.typeSizes.hnsw).toBeGreaterThan(0)
+    expect(stats.typeSizes.vectors).toBeGreaterThan(0)
   })
 
   it('should prioritize frequently accessed items regardless of type', () => {
@@ -52,7 +52,7 @@ describe('UnifiedCache Eviction Scoring', () => {
     }
 
     // Add item with low access count, high rebuild cost
-    cache.set('cold1', { data: 'cold' }, 'hnsw', 100, 50)
+    cache.set('cold1', { data: 'cold' }, 'vectors', 100, 50)
     // Only access once (implicit from set)
 
     // Fill cache
@@ -112,7 +112,7 @@ describe('UnifiedCache Eviction Scoring', () => {
     }
 
     // Add one HNSW with high value
-    cache.set('hnsw1', { data: 'vector' }, 'hnsw', 100, 50)
+    cache.set('hnsw1', { data: 'vector' }, 'vectors', 100, 50)
     for (let i = 0; i < 50; i++) {
       cache.getSync('hnsw1')
     }
@@ -196,8 +196,8 @@ describe('UnifiedCache Eviction Scoring', () => {
     }
 
     // Add HNSW vectors (expensive to rebuild)
-    cache.set('hnsw1', { vector: [1, 2, 3] }, 'hnsw', 100, 50)
-    cache.set('hnsw2', { vector: [4, 5, 6] }, 'hnsw', 100, 50)
+    cache.set('hnsw1', { vector: [1, 2, 3] }, 'vectors', 100, 50)
+    cache.set('hnsw2', { vector: [4, 5, 6] }, 'vectors', 100, 50)
 
     // High access to HNSW
     for (let i = 0; i < 100; i++) {