fix: cold-open no longer re-derives durable indexes — complete the readiness contract for all three providers

A production deployment measured ~48 seconds on EVERY reopen of an
11k-entity brain. Root cause: brainy's rebuild gate decided from in-memory
size()/count, which read 0 for a durable-but-not-resident index, so it
re-read every entity file to rebuild from scratch. At GA we gave only the
GRAPH provider a readiness contract (init() eager cold-load + isReady()
honest signal) so it would never eat that spurious rebuild; the vector and
metadata providers never got it, and brainy never even eager-inited the
vector provider.

Complete the contract symmetrically:

- plugin.ts: VectorIndexProvider gains optional init()+isReady();
  MetadataIndexProvider gains isReady() — mirroring GraphIndexProvider.
  Additive and optional; a provider that exposes nothing keeps today's
  behavior.
- brainy.ts: eager-init every provider that exposes init() (after metadata
  init() so the id-mapper is hydrated first), then decide per leg in
  precedence order — migrating (skip) -> epoch drift (rebuild) -> isReady()
  -> a per-leg empty fallback. The old instant fast-path keyed off
  this.index.size()>0, a dishonest proxy that skipped the metadata/graph
  checks whenever the vector was warm and never fired on a real cold process
  anyway; removed.

The per-leg fallbacks differ because "empty" means different things: the JS
vector's rebuild() IS its load, so size()===0 correctly triggers it; the
id-mapper backs metadata, so totalEntries===0 (past the empty-store return)
is a real load failure; but entities do not imply edges, so a graph
size()===0 is a valid empty state, not a load failure.

- The JS graph now COLD-LOADS its durable LSM instead of re-deriving from a
  full canonical verb scan on every boot (baseStorage._initializeGraphIndex
  loads the persisted SSTables via a new GraphAdjacencyIndex.init(); it
  self-heals from canonical only when the durable state is genuinely missing).
  This removes an O(E)-per-open cost every filesystem consumer paid.
- LSMTree.loadManifest loads its SSTables BEFORE publishing the relationship
  count, and resets to an honest-empty state on load failure — a tree can no
  longer claim persisted relationships while holding none (the silent-empty
  cold-load class the query-time guards exist to prevent).

Verified end-to-end against a built brain: a warm reopen (with edges and
edgeless) reloads only the JS vector; the graph and metadata cold-load with
no rebuild, and queries return correct results. New tests in
cold-open-rebuild-gate.test.ts pin the contract (isReady() defers, self-heal
still fires); migration-deference updated to drive size-based deference
through the vector, the leg where empty->rebuild remains correct.

Pairs with the native provider's isReady()/init() implementation — brainy's
gate defers only to a signal the provider exposes.

src/storage/baseStorage.ts

@@ -2429,11 +2429,26 @@ export abstract class BaseStorage extends BaseStorageAdapter {
     // invalidateGraphIndex); on first init Brainy wires it right after.
     this.graphIndex = new GraphAdjacencyIndex(this, {}, this.graphEntityIdResolver)
 
-    // Check if we need to rebuild from existing data
-    const sampleVerbs = await this.getVerbs({ pagination: { limit: 1 } })
-    if (sampleVerbs.items.length > 0) {
-      prodLog.info('Found existing verbs, rebuilding graph index...')
-      await this.graphIndex.rebuild()
+    // Load the PERSISTED adjacency first (LSM manifests + SSTables). A warm
+    // reopen must load the durable index it already built — the previous
+    // "any verb exists → rebuild()" check here re-derived the whole graph
+    // from a full canonical verb scan on EVERY boot, an O(E) cost that
+    // dominated real deployments' startup.
+    await this.graphIndex.init()
+
+    // Self-heal only when the durable state is genuinely missing: canonical
+    // records exist but the loaded index is empty (first open on pre-index
+    // data, a deleted/corrupt _graph dir, or the LSM load failing loud).
+    // One O(1) probe replaces the unconditional O(E) re-derive.
+    if (this.graphIndex.size() === 0) {
+      const sampleVerbs = await this.getVerbs({ pagination: { limit: 1 } })
+      if (sampleVerbs.items.length > 0) {
+        prodLog.warn(
+          'GraphAdjacencyIndex: canonical verbs exist but the persisted adjacency is empty — ' +
+            'rebuilding from storage (one-time self-heal).'
+        )
+        await this.graphIndex.rebuild()
+      }
     }
 
     return this.graphIndex