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/brainy.ts

@@ -912,6 +912,19 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         this.graphIndex = graphIndex
       }
 
+      // Eager graph cold-load (cor contract). The native graph provider lazily
+      // defers loading its source→target adjacency, so isReady() would report
+      // false at the rebuild gate below and force a spurious rebuild (failing the
+      // §7.1 rebuild()==0 acceptance). Trigger the eager cold-load now — AFTER
+      // metadataIndex.init() above (the id-mapper is hydrated, so a native int
+      // adjacency resolves endpoints through it: the CTX-BR-RESTORE-REBUILD order)
+      // and BEFORE rebuildIndexesIfNeeded. Optional: the JS graph has no init()
+      // and self-loads its adjacency on demand.
+      const graphWithInit = this.graphIndex as { init?: () => Promise<void> }
+      if (typeof graphWithInit.init === 'function') {
+        await graphWithInit.init()
+      }
+
       // 8.0 u64 contract: thread the shared UUID ↔ int resolver into the JS
       // graph index and the storage layer's verb read paths.
       this.wireGraphIdResolver()