Replace the ISS-011 auto-slow with pause-on-landing: when the last plane
in flight lands, tickOnce sets sim_state.paused=true WITHOUT touching
timeScale (speed preserved). flyPlane() now clears paused before
persisting so starting a flight resumes a paused sim — inside runAction(),
tick + unpause + dispatch are atomic under the sim lock.
Tests: engine.test.ts auto-slow block rewritten for pause semantics; new
flyPlane test asserts dispatch clears paused with speed untouched. E2E:
auto-slow.spec.ts renamed to pause-on-landing.spec.ts asserting the new
behavior (paused=true AND timeScale still 1000 after landing; /fly clears
paused). Cross-cutting fix: every per-spec setScale helper now POSTs
{timeScale, paused:false} — a scale-only fast-forward after a landing would
otherwise leave the clock frozen. Deliberate mid-test pauses untouched.
