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/environment.ts

@@ -4,6 +4,7 @@
 
 /**
  * Check if code is running in a browser environment
+ * @deprecated Browser support is deprecated and will be removed in v8.0. Use Node.js or Bun instead.
  */
 export function isBrowser(): boolean {
   return typeof window !== 'undefined' && typeof document !== 'undefined'
@@ -39,6 +40,7 @@ export function isWebWorker(): boolean {
 
 /**
  * Check if Web Workers are available in the current environment
+ * @deprecated Browser support is deprecated and will be removed in v8.0. Use Node.js Worker Threads instead.
  */
 export function areWebWorkersAvailable(): boolean {
   return isBrowser() && typeof Worker !== 'undefined'