feat(8.0): brain.graph.subgraph() + native-provider routing

Introduces the brain.graph namespace and its first op, subgraph() — bounded
multi-hop neighborhood extraction returning a hydrated GraphView (nodes with hop
depth + type/subtype, edges as Relations, a truncated flag). The one-call answer
to 'show me everything around this node, N hops out' the graph-viz path needs.

- brain.graph.subgraph(seeds, { depth, direction, type, subtype, includeInternal,
  includeSystem, maxNodes, maxEdges, hydrateNodes }). seeds: id or id[].
- Feature-detect routing: when a GraphAccelerationProvider is registered
  (isGraphAccelerationProvider on the 'graphAcceleration' provider) it routes to
  native traverse() and hydrates the columnar Subgraph (node ints↔ids paired with
  depth position-preserving, edge verb ints → Relations); otherwise a pure-TS BFS
  fallback expands each frontier through the O(degree) related() adjacency.
- Both paths produce the identical GraphView, so CI exercises the shape via the
  fallback; the native path is cross-layer-tested against cor's provider.
- New public types: GraphView, GraphNode, SubgraphOptions, GraphApi (the surface
  grows: export/rank/path/communities follow). isGraphAccelerationProvider guard
  added to plugin.ts.

Test: graph-subgraph.test.ts — depth tracking, direction, type filter, default-
hides-internal, node hydration on/off, maxNodes truncation, multi-seed.

src/plugin.ts

@@ -681,6 +681,27 @@ export function isVersionedIndexProvider(
   )
 }
 
+/**
+ * @description Feature-detect an optional {@link GraphAccelerationProvider} (the
+ * native graph engine). Brainy probes the registered `'graphAcceleration'`
+ * provider with this: present ⇒ `brain.graph.*` routes to native `traverse` /
+ * cursor / analytics; absent ⇒ Brainy serves the same operations from its
+ * pure-TS adjacency fallback. Checks the two anchor methods (`traverse` +
+ * `graphCursorOpen`) — a partial implementation that lacks them is treated as
+ * absent (fall back rather than crash).
+ * @param candidate - The value registered under `'graphAcceleration'`, or undefined.
+ * @returns Whether `candidate` is a usable {@link GraphAccelerationProvider}.
+ */
+export function isGraphAccelerationProvider(
+  candidate: unknown
+): candidate is GraphAccelerationProvider {
+  if (candidate === null || typeof candidate !== 'object') {
+    return false
+  }
+  const c = candidate as Record<string, unknown>
+  return typeof c.traverse === 'function' && typeof c.graphCursorOpen === 'function'
+}
+
 /**
  * The object returned by the `'vector'` provider factory — Brainy's vector
  * index contract. Implementations include Brainy's own JS HNSW index and any