Why: Late source requests must not replace a newer preview or survive conversation navigation.
Evidence: Frontend typecheck, 221 unit tests, knip ratchet and build passed; delayed-response regressions reproduced the old failure.
Impact: The existing store owns source requests and invalidates stale responses on preview and context changes.
Refs: Agent-System-Dev feature-fast-slow-u2-u3-plan; independent review finding 4