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/errors/brainyError.ts

@@ -14,6 +14,7 @@ export type BrainyErrorType =
   | 'FIELD_NOT_INDEXED'
   | 'GRAPH_INDEX_NOT_READY'
   | 'METADATA_INDEX_NOT_READY'
+  | 'VECTOR_INDEX_NOT_READY'
   | 'MIGRATION_IN_PROGRESS'
 
 /**
@@ -280,6 +281,32 @@ export class MetadataIndexNotReadyError extends BrainyError {
     }
 }
 
+/**
+ * Thrown when the vector index reports vectors (`size() > 0` or, on a native
+ * provider, `isReady() === false`) but cannot return a KNOWN persisted vector
+ * even after a rebuild — i.e. the semantic serving structure did not load on a
+ * cold open and could not be restored. The vector-search counterpart of
+ * {@link GraphIndexNotReadyError} / {@link MetadataIndexNotReadyError}: it
+ * replaces the silent-empty failure mode (a cold `find({ query })` returning
+ * `[]` indistinguishable from "no similar data") with a loud, catchable error,
+ * so a consumer never renders "nothing found" over data that is simply
+ * not-yet-warm.
+ *
+ * Detected once per brain by a known-vector serving probe on the first
+ * semantic / proximity `find()`; brainy self-heals (rebuilds the index from the
+ * canonical records) first and only raises this if the rebuild still cannot
+ * serve the known vector.
+ */
+export class VectorIndexNotReadyError extends BrainyError {
+    constructor(message: string, originalError?: Error) {
+        super(message, 'VECTOR_INDEX_NOT_READY', false, originalError)
+        this.name = 'VectorIndexNotReadyError'
+        if (Error.captureStackTrace) {
+            Error.captureStackTrace(this, VectorIndexNotReadyError)
+        }
+    }
+}
+
 /**
  * Thrown when a data-plane read or write is issued against a brain that is
  * running its one-time, automatic 7.x → 8.0 on-disk upgrade — the coordinated