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/graph/graphAdjacencyIndex.ts

@@ -12,6 +12,7 @@ import { GraphVerb, StorageAdapter } from '../coreTypes.js'
 import { UnifiedCache, getGlobalCache } from '../utils/unifiedCache.js'
 import { prodLog } from '../utils/logger.js'
 import { LSMTree } from './lsm/LSMTree.js'
+import type { GraphIndexProvider } from '../plugin.js'
 
 export interface GraphIndexConfig {
   maxIndexSize?: number      // Default: 100000
@@ -36,7 +37,7 @@ export interface GraphIndexStats {
  * Memory efficient: 385x less memory (1.3GB vs 500GB for 1B relationships)
  * Performance: Sub-5ms neighbor lookups with bloom filter optimization
  */
-export class GraphAdjacencyIndex {
+export class GraphAdjacencyIndex implements GraphIndexProvider {
   // LSM-tree storage for outgoing and incoming edges
   private lsmTreeSource: LSMTree  // sourceId -> targetIds (outgoing edges)
   private lsmTreeTarget: LSMTree  // targetId -> sourceIds (incoming edges)