Companion to 7.33.4's graph cold-read guard, now for the metadata field index. A
downstream deployment reported find({where}) returning a silent [] on a
freshly-opened brain (a native metadata provider that reports data but hasn't
loaded its field postings), blanking filtered pages after every restart.
verifyMetadataLive() — one-shot per brain on the first filtered find(): probe a
known persisted field value; if served, live (one O(1) probe on a warm brain). If
not, rebuild from canonical + re-probe; if still unserved, throw the new exported
MetadataIndexNotReadyError rather than let a silent [] pass. Inconclusive cases
(empty store, no plain field, shared-store foreign entity) resolve to live, no
false rebuild. Also exports BrainyError + GraphIndexNotReadyError (were internal).
The JS index cold-loads correctly; this guards the native path (durable cure is
cortex-side, same shape as the graph 2.7.8 cure). 4 unit tests. tsc 0, guard 4/4,
full unit 1538 pass (+1 pre-existing flaky VFS perf test, green in isolation).
