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/main_menu.gd

@@ -50,6 +50,14 @@ var _is_authenticating: bool = false
 # ─── Lifecycle ────────────────────────────────────────────────────────────────
 
 func _ready() -> void:
+	var v := Label.new()
+	v.text = "build %s" % ProjectSettings.get_setting("application/config/version", "dev")
+	v.add_theme_color_override("font_color", Color(1, 1, 1, 0.4))
+	v.add_theme_font_size_override("font_size", 12)
+	v.set_anchors_and_offsets_preset(Control.PRESET_BOTTOM_RIGHT)
+	v.position = Vector2(-140, -22)
+	add_child(v)
+
 	# Cache node references.
 	_auth_buttons_row = $CenterContainer/VBoxContainer/AuthButtonsRow
 	_login_button = $CenterContainer/VBoxContainer/AuthButtonsRow/LoginButton