Add virtua hand lighting up floor buttons!

- hand sweeps down the button panel at game start
- button panel now spawns its floor buttons procedurally via button_panel.gd
- elevator_panel waits for the new intro_finished signal before starting floor 1
- removes the stray ShaftStrip from hud.tscn
- cleaned up timing but henry will probably want to tune further

scenes/left_hand.tscn

@@ -0,0 +1,11 @@
+[gd_scene format=3 uid="uid://cl3hb1nm4xy8t"]
+
+[ext_resource type="Texture2D" path="res://images/virtua_hand_left.png" id="1_lhand"]
+[ext_resource type="Script" path="res://scenes/left_hand.gd" id="2_lhscr"]
+
+[node name="LeftHand" type="Sprite3D"]
+billboard = 1
+texture_filter = 0
+alpha_cut = 1
+texture = ExtResource("1_lhand")
+script = ExtResource("2_lhscr")