Introduce a probe-owner heartbeat and periodic reaper to improve MTProxy probe liveness.
- Add ConnectionSocket::mtProxyProbeHeartbeat (declared in .h/.cpp) and call it at key handshake milestones (socket_connected, client_hello_sent) to touch the probe owner.
- Make ConnectionsManager::select call MtProxyProbeCoordinator::reapExpired once per second; add lastProbeReapMs field and include the coordinator header.
- Update Tools/check_mtproxy_probe_coordinator.py to enforce the new invariants (bounded PROBING lifetime, join budget anchoring, touchOwner/reapExpired presence and reaper placement).
These changes ensure owners have bounded lifetimes and are kept alive by handshakes, and stalled owners are reclaimed on the network-thread tick.
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