perf: enable Android emulator Quick Boot - #359

Description

Owned Android AVDs cold-boot on every restart because Stim disables Quick Boot loading and saving. The measurements in #284 found about 22 seconds of avoidable startup time.

The flags were added in PR #93 to reduce AVD storage (original commit), before Stim measured and configured owned-AVD userdata.

Fixes #284.

Solution

Use Android Emulator's default Quick Boot policy for owned AVDs. Displayless Linux retains disabled snapshots because its software renderer makes them unreliable. New or invalid snapshots fall back to a cold boot; Android invalidates snapshots after emulator, system-image, or AVD-setting changes (Android documentation).

Teardown preserves the bounded guest-write flush from #357 and waits for the emulator process to exit after saving its snapshot. The 60-second shutdown budget includes sync and the adb command. Invalid PID locks, unresolved live emulators, shutdown timeouts, and pre-deletion restarts prevent deletion and keep the ownership record. Windows can expose an empty PID file before writing its PID, so a present invalid lock refuses cleanup (upstream lock implementation).

The storage policy keeps one automatic snapshot. New owned AVDs retain the configurable 8 GiB default data partition; existing AVDs keep their size. worktree remove deletes the whole AVD, and gc reports stale or orphaned owned-AVD sizes. The guide and benchmark control prompt describe the same policy.

Test plan

• 288 focused Android, teardown, ownership, and guide tests passed. Shutdown deadline and invalid-lock regressions both failed when their fixes were temporarily removed.
• Format, lint, build, typecheck, knip, website build/typecheck, all 3,662 unit tests, and 24 end-to-end tests passed.
• Created an isolated Stim-owned ARM64 API 34 AVD with Android Emulator 35.6.11. The first snapshot-enabled cold boot took 13.5 seconds; centralized shutdown waited 2.5 seconds for process exit and saved default_boot. The next boot took 5.6 seconds, logged a successful snapshot load, and restored a marker written immediately before shutdown. These are one local run's observations, not a benchmark series.
• Centralized teardown then deleted that AVD and cleared its isolated ownership record. Its measured footprint was 2.46 GiB before deletion. No pre-existing device was changed.