feat(plugin): DiskAnnProvider contract + HNSWConfig.type/diskann knobs

Adds the plugin-side surface for the new billion-scale index option:

- plugin.ts: DiskAnnProvider type alias (mirrors HnswProvider's shape
  so cortex's DiskANN wrapper is a drop-in for the 'hnsw' factory).
- coreTypes.ts: HNSWConfig.type ('hnsw' | 'diskann') for explicit
  engine selection, plus HNSWConfig.diskann for the build-time tuning
  knobs (pqM, pqKsub, maxDegree, searchListSize, alpha, mmap
  adjacency).

No algorithm code here — the implementation lives in the cortex
plugin. Brainy without cortex continues to use HNSW exactly as before.

src/plugin.ts

@@ -216,6 +216,31 @@ export interface HnswProvider {
   getPersistMode(): 'immediate' | 'deferred'
 }
 
+/**
+ * The `'diskann'` provider — a billion-scale alternative to HNSW backed
+ * by cortex's pure-Rust Vamana + Product Quantization implementation
+ * (see ADR-002 in the cortex repo).
+ *
+ * Structurally a drop-in for [`HnswProvider`]: brainy calls the same
+ * `addItem` / `search` / `rebuild` surface and never needs to know
+ * which index is underneath. The differences are operational:
+ *
+ * - **Memory footprint**: ~16 GB RAM at 1 B vectors (PQ codes only).
+ *   HNSW would need ~1.5 TB to keep the full vectors resident.
+ * - **Build model**: build-once, query-many. Dynamic insertions buffer
+ *   to an in-memory delta brute-forced alongside the main index;
+ *   `rebuild()` folds them in.
+ * - **Storage**: requires a local filesystem path (NVMe SSD for the
+ *   published latency numbers). Cloud-storage adapters continue using
+ *   HNSW.
+ *
+ * Engagement is automatic when the cortex provider is registered, the
+ * storage adapter exposes `getBinaryBlobPath`, and the metadata index
+ * has a stable `idMapper`. Users force the legacy index via
+ * `config.index.type = 'hnsw'`.
+ */
+export type DiskAnnProvider = HnswProvider
+
 /**
  * The `'entityIdMapper'` provider — a drop-in for `EntityIdMapper`. Injected
  * into the TypeScript `MetadataIndexManager` when a native metadata index is