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/cli.py

@@ -23,14 +23,27 @@ def main() -> None:
         default=30.0,
         help="Seconds before a node is removed from the registry after missed heartbeat",
     )
+    start_cmd.add_argument(
+        "--cluster-peers",
+        default="",
+        help="Comma-separated URLs of peer tracker nodes (enables Raft cluster mode)",
+    )
+    start_cmd.add_argument(
+        "--self-url",
+        default=None,
+        help="This tracker's own URL as seen by peers (auto-derived from --host/--port if omitted)",
+    )
 
     args = parser.parse_args()
 
     if args.command == "start":
+        cluster_peers = [u.strip() for u in args.cluster_peers.split(",") if u.strip()]
         server = TrackerServer(
             host=args.host,
             port=args.port,
             heartbeat_timeout=args.heartbeat_timeout,
+            cluster_peers=cluster_peers or None,
+            cluster_self_url=args.self_url,
         )
         port = server.start()
         print(f"meshnet-tracker listening on http://{args.host}:{port}", flush=True)