feat: wire plugin system with provider resolution, storage factories, and browser deprecation

- Wire PluginRegistry into Brainy init() with provider resolution for distance,
  metadataIndex, graphIndex, embeddings, roaring, msgpack, and storage adapters
- Add setupStorage() factory that resolves storage:* providers from plugins before
  falling back to built-in createStorage()
- Export internals API (setGlobalCache, UnifiedCache, EntityIdMapper, etc.) for
  cortex plugin consumption
- Add plugin.test.ts verifying registration, activation, and provider resolution
- Deprecate browser support (OPFS, Web Workers, WASM embeddings) with warnings
  in preparation for v8.0 server-only release
- FileSystemStorage: fix setupStorage resolution for mmap-filesystem provider

src/storage/adapters/fileSystemStorage.ts

@@ -78,7 +78,7 @@ export class FileSystemStorage extends BaseStorage {
   private readonly SHARDING_DEPTH = 1 as const
   private readonly MAX_SHARDS = 256 // Hex range: 00-ff
   private cachedShardingDepth: number = this.SHARDING_DEPTH // Always use fixed depth
-  private rootDir: string
+  protected rootDir: string
   private nounsDir!: string
   private verbsDir!: string
   private metadataDir!: string