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/entityIdMapper.ts

@@ -14,6 +14,7 @@
  */
 
 import type { StorageAdapter } from '../coreTypes.js'
+import type { EntityIdMapperProvider } from '../plugin.js'
 
 export interface EntityIdMapperOptions {
   storage: StorageAdapter
@@ -27,9 +28,12 @@ export interface EntityIdMapperData {
 }
 
 /**
- * Maps entity UUIDs to integer IDs for use with Roaring Bitmaps
+ * Maps entity UUIDs to integer IDs for use with Roaring Bitmaps.
+ *
+ * Implements {@link EntityIdMapperProvider}: the surface a registered
+ * `'entityIdMapper'` provider (e.g. Cortex's native mapper) must also satisfy.
  */
-export class EntityIdMapper {
+export class EntityIdMapper implements EntityIdMapperProvider {
   private storage: StorageAdapter
   private storageKey: string