fix: honest index readiness — no silently-empty queries on a cold index

Closes the "dishonest readiness proxy" anti-pattern (Pattern A): size()>0 /
isInitialized were treated as "this index serves queries", but a cold native
index can load its COUNT before its SERVING structure, so a query returned a
silent [] indistinguishable from "no such data". A shared assessIndexReadiness()
now reads only the provider's honest isReady() signal (never size()), applied at
every site:

- Vector: a one-shot verifyVectorLive() guard on the semantic/proximity search
  path (a pure semantic find({query}) has no filter, so nothing guarded it). It
  prefers isReady(), else a known-vector self-match probe; self-heals via rebuild
  or throws the new VectorIndexNotReadyError instead of a silent [].
- Graph: getVerbsBySource/ByTarget skip the fast path when the provider reports
  not-ready (falling to the canonical shard scan), plus a one-shot probe that
  self-heals a no-isReady provider whose adjacency did not cold-load.
- getIndexStatus(): folds in per-index honest `ready` (making `populated`
  honest) + rebuildFailed/rebuildError/degradedIds, so a readiness probe never
  200s a brain that is still warming up or degraded.

Unblocked by the native providers now reporting serving-truth (graph via
SSTable-residency readiness, vector via durableBaseLoadFailed). New export:
VectorIndexNotReadyError. getIndexStatus gains additive fields. No breaking API.
13 new tests; existing readiness guards green.

src/index.ts

@@ -150,7 +150,7 @@ export { EntityNotFoundError, RelationNotFoundError } from './errors/notFound.js
 
 // Base error + typed migration-lock error — thrown by any data-plane call while a
 // brain runs its one-time 7.x→8.0 upgrade; catch to answer HTTP 503 + Retry-After.
-export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError } from './errors/brainyError.js'
+export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError, VectorIndexNotReadyError } from './errors/brainyError.js'
 export type { BrainyErrorType } from './errors/brainyError.js'
 
 // ============= 8.0 Db API — generational MVCC =============