#1979 S6: fail fast on a dead pick — demote on real failure + composer warning with one-click switch #1989

Parent: #1979. Blocked by: #1985, #1986.

When a real request fails with a reachability-class error, demote the model via S2 POST …/demote. If the session's chosen model is unreachable, the composer shows a named warning with a one-click switch to the recommended equivalent. It must never fail silently mid-turn. Files: overlay context/local.tsx, pages/session/composer/session-composer-controls.ts, prompt-input*.tsx.

AC (RED first):

• Pure-logic tests: a denied error demotes, the warning model picks the recommended equivalent, and a transient error (rate-limited, TLS) does not demote.