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/utils/unifiedCache.ts

@@ -602,6 +602,10 @@ export function getGlobalCache(config?: UnifiedCacheConfig): UnifiedCache {
   return globalCache
 }
 
+export function setGlobalCache(cache: UnifiedCache): void {
+  globalCache = cache
+}
+
 export function clearGlobalCache(): void {
   if (globalCache) {
     globalCache.clear()