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/graph/lsm/LSMTree.ts

@@ -547,13 +547,26 @@ export class LSMTree {
         const data = metadata.data as PersistedManifestData
         this.manifest.sstables = new Map(Object.entries(data.sstables || {}))
         this.manifest.lastCompaction = data.lastCompaction || Date.now()
-        this.manifest.totalRelationships = data.totalRelationships || 0
 
-        // Load SSTables from storage
+        // Load SSTables from storage BEFORE publishing the persisted count.
+        // If the SSTable load throws, `size()` must keep reporting 0 — a tree
+        // that claims its persisted relationships while holding none serves
+        // silent-empty traversals as truth (the cold-load swallow class), and
+        // downstream self-heal keys off the honest 0.
         await this.loadSSTables()
+        this.manifest.totalRelationships = data.totalRelationships || 0
       }
     } catch (error) {
-      prodLog.debug('LSMTree: No existing manifest found, starting fresh')
+      // Reset anything partially loaded — an honest empty tree triggers the
+      // rebuild/self-heal paths; a half-loaded one masks them. (An absent
+      // manifest on a fresh store also lands here: empty is correct.)
+      this.manifest.sstables = new Map()
+      this.manifest.totalRelationships = 0
+      this.sstablesByLevel.clear()
+      prodLog.debug(
+        `LSMTree(${this.config.storagePrefix}): no loadable manifest/SSTables — starting empty ` +
+          `(${error instanceof Error ? error.message : String(error)})`
+      )
     }
   }