Node now sends cumulative stats in heartbeat body:
total_requests, failed_requests, queue_depth, uptime_seconds, status
Stats are tracked thread-safely in _TorchHTTPServer; buffered locally during
outage streak and flushed on next successful heartbeat.
Tracker stores stats on _NodeEntry (total_requests, failed_requests,
queue_depth, uptime_seconds, status, heartbeats_expected/received) and
returns new_assignment in heartbeat response when pending_new_assignment is set.
Node logs received new_assignment (hot-reload wired in US-026).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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