Keep iOS simulator runner hot across session close #1013

Problem

#1010/#1011 made the XCUITest runner survive open --relaunch and daemon restarts, but session close still tears it down: src/daemon/session-teardown.ts → stopIosRunnerSession. Closing a session and opening a new one on the same simulator pays the full ~5s xcodebuild test-without-building ramp for no benefit — the runner is device-scoped, app-agnostic infrastructure; the session is a logical construct.

Proposal

Let a healthy simulator runner outlive session close (same daemon), reusing the existing readiness-preflight + invalidate/restart recovery for staleness — the identical argument that carried the relaunch keep-hot change.

Constraints (from the #1011 review/verification)

• Proxy lease expiry flows through the same teardown (cleanupExpiredLeasedSession → session teardown). Decide explicitly whether expired-tenant cleanup keeps killing the runner (conservative) or can also keep it hot given lease admission gates device access anyway.
• Scoped simulator-set runners must keep their teardown — the XCTestDevices redirect symlink/lock lifetime is session-bound (see the scoped-set fix in perf: overlap iOS runner prewarm, seed booted-memo, hand runners off across daemon restarts #1011).
• Consider an idle policy for a runner left behind after close (the runner self-expires via XCTWaiter after 24h; that may be enough).

This changes what close means operationally, so it deserves its own semantics discussion — filing separately from the perf PRs on purpose.