ensureEmbeddingReady admission

Chose finite, guarded readiness admission over retaining ownership behind local-worker warmup because an unbounded stuck warmup wedges readiness and document admission forever. After admission, re-read provider identity, pass the readiness guard into the pool request, retire an assigned worker when the deadline fires, and map pool cancellation to fixed EmbeddingAbortError; never accept late output. Release ownership in finally on disappearance, OOM, timeout, rejection, or cancellation.