feat(8.0): GraphAccelerationProvider contract — the native graph-engine seam

Defines the optional native graph-acceleration provider (cor 3.0) that
brainy feature-detects and routes brain.graph.* / related({node}) /
find({connected}) to, falling back to pure-TS adjacency when absent. This is
the published seam cor wires its native build against (mirrors the existing
VersionedIndexProvider pattern: defined in plugin.ts, exported from index.ts,
implemented externally, feature-detected at runtime).

Contract (locked cross-team in the design thread):
- GraphAccelerationProvider: traverse, edgesForNode, graphCursorOpen/Next/Close,
  pageRank, connectedComponents, shortestPath, neighborhoodSample, topByDegree.
  Every read op is generation-aware (optional trailing generation?: bigint) so
  db.asOf(g).graph.* resolves historically; omitted = now.
- Subgraph: columnar wire format (BigInt64Array node/edge columns, Uint16Array
  edgeTypes, optional Uint8Array nodeDepth, truncated flag) — parallel typed
  arrays, never array-of-objects; brainy maps u64<->UUID lazily for rendered rows.
- OpaqueIdSet: a find() universe forwarded opaquely into traverse/search for the
  zero-crossing query->expand fusion (brainy never inspects it; the provider
  version-tags + validates the envelope).
- VectorIndexProvider.search gains allowedIds?: OpaqueIdSet | ReadonlySet<string>
  (predicate pushdown — recovers filtered recall lost to post-filtering).
- EntityIdMapperProvider gains entityIntsToUuids(BigInt64Array) — the bigint batch
  reverse resolver for Subgraph.nodes; implemented on the JS mapper (TS fallback).

All new public types exported from index.ts. Test: entityIntsToUuids round-trip
+ order-preservation + empty-int sentinel.

src/index.ts

@@ -180,6 +180,28 @@ export type {
 // Optional provider capability for generation-aware native indexes
 export { isVersionedIndexProvider } from './plugin.js'
 export type { VersionedIndexProvider } from './plugin.js'
+// Optional native graph-acceleration engine (cor 3.0) — the published provider
+// contract + its columnar wire types. Brainy feature-detects an implementation
+// and falls back to its pure-TS adjacency when absent.
+export type {
+  GraphAccelerationProvider,
+  Subgraph,
+  OpaqueIdSet,
+  GraphTraversalDirection,
+  TraverseOptions,
+  EdgesForNodeOptions,
+  GraphCursorHandle,
+  GraphCursorOptions,
+  GraphCursorChunk,
+  GraphScores,
+  GraphComponents,
+  GraphPath,
+  PageRankOptions,
+  ComponentsOptions,
+  ShortestPathOptions,
+  NeighborhoodSampleOptions,
+  TopByDegreeOptions
+} from './plugin.js'
 
 // Export embedding functionality
 import {