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/metadataIndex.ts

@@ -6,6 +6,7 @@
 
 import { StorageAdapter, resolveEntityField } from '../coreTypes.js'
 import { ColumnStore } from '../indexes/columnStore/ColumnStore.js'
+import type { MetadataIndexProvider } from '../plugin.js'
 import { MetadataIndexCache, MetadataIndexCacheConfig } from './metadataIndexCache.js'
 import { compareCodePoints } from './collation.js'
 import { prodLog } from './logger.js'
@@ -103,7 +104,12 @@ interface FieldStats {
   normalizationStrategy?: 'none' | 'precision' | 'bucket'
 }
 
-export class MetadataIndexManager {
+/**
+ * Implements {@link MetadataIndexProvider}: the metadata-index surface Brainy
+ * calls on whatever the `'metadataIndex'` provider resolves to (its own
+ * manager, or Cortex's native Rust engine).
+ */
+export class MetadataIndexManager implements MetadataIndexProvider {
   private storage: StorageAdapter
   private config: Required<MetadataIndexConfig>
   private isRebuilding = false