refactor: remove 3,700+ LOC of unused HNSW implementations

Delete dead code island that was never instantiated in production:
- OptimizedHNSWIndex (430 LOC)
- PartitionedHNSWIndex (412 LOC)
- DistributedSearchSystem (635 LOC)
- ScaledHNSWSystem (744 LOC)
- HNSWIndexOptimized (585 LOC)
- brainy-backup.ts stale example (903 LOC)

Also upgrades entry point recovery from O(n) to O(1) using existing
highLevelNodes index structure.

Production uses only: HNSWIndex (memory) and TypeAwareHNSWIndex (persistent)

src/index.ts

@@ -346,14 +346,10 @@ import type {
   StorageAdapter
 } from './coreTypes.js'
 
-// Export HNSW index and optimized version
+// Export HNSW index
 import { HNSWIndex } from './hnsw/hnswIndex.js'
-import {
-  HNSWIndexOptimized,
-  HNSWOptimizedConfig
-} from './hnsw/hnswIndexOptimized.js'
 
-export { HNSWIndex, HNSWIndexOptimized }
+export { HNSWIndex }
 
 export type {
   Vector,
@@ -365,7 +361,6 @@ export type {
   HNSWNoun,
   HNSWVerb,
   HNSWConfig,
-  HNSWOptimizedConfig,
   StorageAdapter
 }