Adds a Healthy/Draining/Restarting lifecycle gate. `acquire()` parks while
a restart is in progress; `coordinated_restart(deadline)` blocks new
acquires, drains in-flight leases (bounded, then forces), closes +
relaunches Chromium (re-running on_launch → re-inject session + probe),
then resumes parked acquirers. Concurrent restart requests collapse into
one relaunch; the phase always returns to Healthy so a failed relaunch
never wedges acquisition.
The metadata pass cooperates via is_restart_pending() at its per-manga
checkpoint, yielding its long-lived lease (recovery sweep next tick)
instead of stalling the drain.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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