Fix ESC-crashes-the-game, avatar facing backward, invisible build stamp

ESC: player.gd changed scene directly while game.gd._exit_tree closed the
socket, which re-fired server_disconnected -> _on_server_lost ran mid-
teardown (add_child on an exiting node, await on a null get_tree(), a
second change_scene_to_file). Moved ESC to game.gd as one ordered path:
_leave_world() sets a _leaving guard, disconnects the _on_server_lost
handlers before closing the connection, then changes scene. _on_server_lost
and _exit_tree now respect the guard. Verified headless: ESC -> world_list,
no errors; 2p multiplayer still connects.

Avatar faced +Z but a Godot node's forward is -Z, so walking forward
showed the character its own face. Rebuilt the placeholder facing -Z (face
plates, eyes, jaw, toes, and front-mounted attachments all moved to
negative Z; backpack/ponytail to +Z). The procedural walk was already
written for -Z forward, so it's unchanged.

Build stamp was there but tiny/low-contrast bottom-left -- bumped to 20px
opaque yellow with a heavy outline, moved to the top-left strip.

limb_animator: guard _process with is_instance_valid(_skel) so a frame
landing during avatar teardown can't touch a freed skeleton.

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

scripts/avatar/limb_animator.gd

@@ -42,7 +42,7 @@ func setup(skeleton: Skeleton3D) -> void:
 
 
 func _process(delta: float) -> void:
-	if _skel == null or _idx.is_empty():
+	if not is_instance_valid(_skel) or _idx.is_empty():
 		return
 
 	var moving := speed > MOVE_EPS