feat: export provider contracts for the plugin surface brainy consumes

Native accelerators (cortex) register providers for metadataIndex, graphIndex,
hnsw, entityIdMapper, cache, columnStore, and aggregation. Until now the exact
method/property surface brainy calls on each was implicit — a provider could
drop a member brainy depends on and only fail at runtime when that path ran.

This defines and exports the provider contracts from the stable
@soulcraft/brainy/plugin entrypoint:
- MetadataIndexProvider, GraphIndexProvider, HnswProvider,
  EntityIdMapperProvider, CacheProvider — each typed as exactly the surface
  brainy calls (optional/feature-detected members like HNSW setPersistMode and
  enableCOW are intentionally excluded).
- Re-exports ColumnStoreProvider and AggregationProvider (and the aggregate
  types) from the same entrypoint so a plugin author can import the whole
  provider surface from one place.

Brainy's own baseline classes now `implements` these contracts
(MetadataIndexManager, GraphAdjacencyIndex, HNSWIndex, EntityIdMapper,
UnifiedCache), so the interfaces can never silently diverge from what brainy
ships — and any provider that declares `implements` gets a compile error the
moment brainy starts requiring a new member.

The embeddings/embedBatch providers stay typed by the existing EmbeddingFunction
(no duplicate interface added). Type-only changes; no runtime behavior change.

src/utils/unifiedCache.ts

@@ -17,6 +17,7 @@ import {
   type MemoryInfo,
   type CacheAllocationStrategy
 } from './memoryDetection.js'
+import type { CacheProvider } from '../plugin.js'
 
 export interface CacheItem {
   key: string
@@ -57,7 +58,14 @@ export interface UnifiedCacheConfig {
   memoryCheckInterval?: number
 }
 
-export class UnifiedCache {
+/**
+ * Single cost-aware cache for HNSW and MetadataIndex.
+ *
+ * Implements {@link CacheProvider}: the surface Brainy calls on whatever cache
+ * is installed as the global cache (its own instance, or a registered
+ * `'cache'` provider such as Cortex's native eviction engine).
+ */
+export class UnifiedCache implements CacheProvider {
   private cache = new Map<string, CacheItem>()
   private access = new Map<string, number>()  // Access counts
   private loadingPromises = new Map<string, Promise<any>>()