feat(us-019): distributed tracker consensus — Raft assignments + CRDT gossip

raft.py: minimal Raft consensus for shard-assignment log
  - Leader election with random 150–300ms election timeouts
  - AppendEntries log replication; majority commit required
  - RequestVote RPC with log-completeness check
  - Follower registration forwarded to leader via HTTP proxy

gossip.py: LWW CRDT gossip for inference-node heartbeat liveness
  - Each tracker keeps {node_id → wall_clock_timestamp}
  - Merges incoming gossip by taking max per key
  - Pushes snapshot to random peer every 3 seconds

server.py + cli.py:
  - TrackerServer gains cluster_peers + cluster_self_url params
  - New HTTP endpoints: /v1/raft/vote, /v1/raft/append,
    /v1/raft/status, /v1/gossip
  - --cluster-peers and --self-url CLI flags

tests/test_tracker_consensus.py: 6 integration tests
  - Leader elected within 1s
  - Follower registration propagated to all nodes
  - Follower proxy to leader
  - Leader kill → new election within 5s, registrations continue
  - Gossip table updated on heartbeat

92 tests pass, 1 skipped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/tracker/meshnet_tracker/gossip.py

@@ -0,0 +1,88 @@
+"""CRDT gossip for node liveness heartbeats.
+
+Uses a last-write-wins (LWW) register per inference node: each tracker node
+keeps its own copy of {node_id → last_seen_monotonic_timestamp} and merges
+incoming gossip by taking the max per key.  This is eventually consistent —
+a heartbeat received by one tracker propagates to all others within a few
+gossip intervals.
+
+Monotonic timestamps are local-clock-relative; for cross-machine gossip the
+caller should use wall-clock seconds (time.time()).  The tracker converts
+monotonic to wall-clock when recording and back when comparing.
+"""
+
+from __future__ import annotations
+
+import json
+import random
+import threading
+import time
+import urllib.request
+
+
+class NodeGossip:
+    """LWW gossip table for inference-node heartbeat timestamps.
+
+    ``record(node_id)`` is called when a node sends a heartbeat to *this*
+    tracker.  ``merge(remote)`` is called when gossip arrives from a peer
+    tracker.  The table is periodically pushed to one random peer.
+    """
+
+    PUSH_INTERVAL = 3.0  # seconds between gossip pushes to a random peer
+
+    def __init__(self, peers: list[str]) -> None:
+        self.peers = list(peers)
+        # Maps node_id → wall-clock seconds of last known heartbeat.
+        self._table: dict[str, float] = {}
+        self._lock = threading.Lock()
+        self._running = False
+
+    def start(self) -> None:
+        self._running = True
+        threading.Thread(target=self._push_loop, daemon=True, name="gossip").start()
+
+    def stop(self) -> None:
+        self._running = False
+
+    def record(self, node_id: str, wall_ts: float | None = None) -> None:
+        """Record a heartbeat for *node_id* at *wall_ts* (default: now)."""
+        ts = wall_ts if wall_ts is not None else time.time()
+        with self._lock:
+            if ts > self._table.get(node_id, 0.0):
+                self._table[node_id] = ts
+
+    def merge(self, remote: dict[str, float]) -> None:
+        """Merge a gossip snapshot from a peer tracker (LWW per key)."""
+        with self._lock:
+            for node_id, ts in remote.items():
+                if ts > self._table.get(node_id, 0.0):
+                    self._table[node_id] = ts
+
+    def last_seen(self, node_id: str) -> float | None:
+        """Return wall-clock timestamp of last known heartbeat, or None."""
+        with self._lock:
+            return self._table.get(node_id)
+
+    def snapshot(self) -> dict[str, float]:
+        with self._lock:
+            return dict(self._table)
+
+    def _push_loop(self) -> None:
+        while self._running:
+            time.sleep(self.PUSH_INTERVAL)
+            if not self.peers:
+                continue
+            peer = random.choice(self.peers)
+            try:
+                snap = self.snapshot()
+                body = json.dumps(snap).encode()
+                req = urllib.request.Request(
+                    f"{peer}/v1/gossip",
+                    data=body,
+                    headers={"Content-Type": "application/json"},
+                    method="POST",
+                )
+                with urllib.request.urlopen(req, timeout=2.0) as r:
+                    r.read()
+            except Exception:
+                pass