feat(us-023): heartbeat stats payload, request counters, dynamic reassignment response

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>

packages/node/meshnet_node/torch_server.py

@@ -46,6 +46,10 @@ class _TorchHTTPServer(http.server.HTTPServer):
         self.tracker_mode = tracker_mode
         self.tracker_url = tracker_url
         self.route_timeout = route_timeout
+        self.total_requests: int = 0
+        self.failed_requests: int = 0
+        self.queue_depth: int = 0
+        self._stats_lock = threading.Lock()
 
 
 class _TorchHandler(http.server.BaseHTTPRequestHandler):
@@ -219,6 +223,21 @@ class _TorchHandler(http.server.BaseHTTPRequestHandler):
 
     def _handle_chat_completions(self) -> None:
         server: _TorchHTTPServer = self.server  # type: ignore[assignment]
+        with server._stats_lock:
+            server.total_requests += 1
+            server.queue_depth += 1
+        try:
+            self._do_chat_completions(server)
+        finally:
+            with server._stats_lock:
+                server.queue_depth -= 1
+
+    def _record_failed_request(self) -> None:
+        server: _TorchHTTPServer = self.server  # type: ignore[assignment]
+        with server._stats_lock:
+            server.failed_requests += 1
+
+    def _do_chat_completions(self, server: "_TorchHTTPServer") -> None:
         body = self._read_json_body()
         if body is None:
             return
@@ -244,6 +263,7 @@ class _TorchHandler(http.server.BaseHTTPRequestHandler):
                     text = server.backend.generate_text(messages, max_tokens, temperature, top_p)
                     self._send_openai_response(text, model_name, False, messages)
             except Exception as exc:
+                self._record_failed_request()
                 self._send_json(500, {"error": f"generation failed: {exc}"})
             return
 
@@ -620,6 +640,18 @@ class TorchNodeServer:
     def forward_chunk_count(self) -> int:
         return self._server.forward_chunk_count if self._server is not None else 0
 
+    @property
+    def total_requests(self) -> int:
+        return self._server.total_requests if self._server is not None else 0
+
+    @property
+    def failed_requests(self) -> int:
+        return self._server.failed_requests if self._server is not None else 0
+
+    @property
+    def queue_depth(self) -> int:
+        return self._server.queue_depth if self._server is not None else 0
+
     def start(self) -> int:
         if self._server is not None:
             raise RuntimeError("TorchNodeServer is already running")