v0.3.0 — response cache + cross-provider fallback

Response cache and cross-provider fallback are now WIRED and live (both opt-in, default OFF — existing configs unchanged). Plus a semantic cache tier (opt-in), a self-contained Prometheus metrics package, and a deterministic local embedder. Reviewed by 3 focused task-reviews + a final whole-branch review; full suite green under -race incl chaos/security/mutation/helixqa. See CHANGELOG.md.