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/internals.ts

@@ -0,0 +1,11 @@
+/**
+ * Internal utilities for first-party plugins.
+ * NOT part of the public API — may change between minor versions.
+ */
+export { getGlobalCache, setGlobalCache, clearGlobalCache, UnifiedCache } from './utils/unifiedCache.js'
+export type { UnifiedCacheConfig, CacheItem } from './utils/unifiedCache.js'
+export { prodLog, createModuleLogger } from './utils/logger.js'
+export { FieldTypeInference, FieldType } from './utils/fieldTypeInference.js'
+export type { FieldTypeInfo } from './utils/fieldTypeInference.js'
+export { EntityIdMapper } from './utils/entityIdMapper.js'
+export type { EntityIdMapperOptions, EntityIdMapperData } from './utils/entityIdMapper.js'