feat(8.0): eager graphIndex.init() before the isReady() rebuild gate

A native graph provider cold-loads its source->target adjacency lazily, so
isReady() would report false at the rebuild gate on a cold open and force a
spurious rebuild (failing the §7.1 rebuild()==0 acceptance). Add an OPTIONAL
GraphIndexProvider.init() (mirrors MetadataIndexProvider.init) and call it during
init, AFTER metadataIndex.init() (id-mapper hydrated first, so a native int
adjacency resolves endpoints through it) and BEFORE rebuildIndexesIfNeeded. The JS
graph index omits init() and self-loads on demand, so the JS path is unchanged.

src/plugin.ts

@@ -264,6 +264,18 @@ export interface GraphIndexProvider {
    */
   isReady?(): boolean
 
+  /**
+   * @description OPTIONAL eager cold-load. Called once during brain init — AFTER
+   * the metadata provider's `init()` (so the id-mapper is hydrated; a native int
+   * adjacency resolves endpoints through it — the id-mapper-before-adjacency
+   * order) and BEFORE the rebuild gate — so a native provider loads its
+   * source→target adjacency from storage and reports `isReady() === true` at the
+   * gate, with no spurious rebuild (the §7.1 `rebuild()==0` acceptance). Mirrors
+   * {@link MetadataIndexProvider.init}. The JS graph index omits it and
+   * self-loads its adjacency on demand.
+   */
+  init?(): Promise<void>
+
   /**
    * @description Entity ints reachable from `id` (1 hop), deduped.
    * @param id - The entity's interned int (from the shared idMapper).