Replace REST-based Discord liveness probe with local WebSocket/heartbeat
state detection. REST success doesn't prove Gateway event delivery — a
half-closed WebSocket can leave Bot.start() alive while REST returns 200.
Now samples ready/open/ACK state and heartbeat latency; consecutive
unhealthy samples emit one retryable fatal code so GatewayRunner rebuilds
the adapter through the existing reconnect path.
Also fixes three lifecycle gaps in the recovery path:
1. asyncio.wait_for() can remain blocked if adapter cleanup swallows
cancellation — now uses bounded asyncio.wait() with task detachment.
2. Multiplexed secondary-profile adapters had no profile-scoped reconnect
owner — now uses one runner-owned reconnect slot per profile.
3. An in-flight turn could send its final text through the disconnected
adapter after a replacement was registered — now resolves the live
same-profile replacement for unsent final responses only (message IDs
never migrate, edits/deletes stay on the old transport).
Adds an opt-in Linux/systemd event-loop watchdog (gateway.systemd_watchdog_seconds,
default 0) for the failure mode where the whole asyncio loop stops making
progress and no in-process liveness task can run. stdlib-only sd_notify,
Type=notify/WatchdogSec generation, READY/STOPPING lifecycle.
Co-authored-by: 王鑫 <wx.xw@bytedance.com>
