Fix multiplayer state: stacked players, ghosts after rejoin; add a build stamp

- Clients now hard-snap to the server's spawn position on the first
  snapshot. player.gd starts everyone on the fixed PlayerSpawn marker but
  the server spreads spawns out -- so one client saw players ~1m apart
  (correct) while the other saw them stacked (its local player still on the
  marker, never corrected because the ~1m drift was under the old 1.5m
  threshold). Threshold also tightened to 0.6m.
- game.gd._exit_tree() closes the ENet connection cleanly when you leave a
  world, so the server fires peer_disconnected and removes your player.
  Before, leaving just changed scene and the server never knew -- everyone
  else kept seeing a frozen "ghost".
- server.gd: once-a-second sweep of players whose peer is no longer
  connected, as a backstop for a missed disconnect.
- Build stamp: checkpoint_ops bakes "<PC> <commit>" into
  application/config/version at export; the menu and in-world HUD show
  "build PC3 <hash>" so playtest reports aren't ambiguous about which
  build is running.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/server.gd

@@ -102,11 +102,27 @@ func _physics_process(_delta: float) -> void:
 	if not is_running:
 		return
 	tick += 1
+	if tick % PHYSICS_HZ == 0:
+		_sweep_stale_players()
 	_broadcast_snapshot()
 	if tick % STATUS_WRITE_EVERY == 0:
 		_write_status()
 
 
+## Backstop for _on_peer_disconnected: once a second, drop any player whose
+## peer is no longer connected (covers a missed disconnect signal, so a
+## rejoining client never sees a frozen ghost of its old self).
+func _sweep_stale_players() -> void:
+	var connected := multiplayer.get_peers()
+	for peer_id in players.keys():
+		if peer_id not in connected:
+			var p: Node = players[peer_id]
+			if is_instance_valid(p):
+				p.queue_free()
+			players.erase(peer_id)
+			print("[Server] swept stale player %d (%d left)" % [peer_id, players.size()])
+
+
 func _broadcast_snapshot() -> void:
 	var snap := {"tick": tick, "players": {}}
 	for peer_id in players: