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. Brainy
had a cold-read guard for GRAPH reads (verifyGraphAdjacencyLive → self-heal or a
loud GraphIndexNotReadyError) but no equivalent for metadata `where`.
Add verifyMetadataLive() — the field-index counterpart, one-shot per brain on the
first filtered find(): probe a known persisted entity's plain field value; if the
index serves it, live (the only cost on a warm brain — one O(1) probe). If not,
the postings didn't load: 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,
migrating provider) resolve to live — never a false rebuild.
The 8.0 open-core JS index already cold-loads correctly (verified: cold where 3/3,
cold related 2/2) — this guards the NATIVE path, where the durable cold-load cure
is cortex-side (same shape as the graph 2.7.8 cure). 4 unit tests (warm no-rebuild,
self-heal, loud-fail, no-filter-no-probe). Gates: typecheck 0, build 0, test:unit
1757/1757.
