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/button_panel.gd

@@ -0,0 +1,18 @@
+extends Node3D
+
+const BUTTON_SCENE = preload("res://scenes/floor_button.tscn")
+const BUTTON_COUNT := 10
+const Y_TOP := 8.49
+const Y_BOTTOM := -4.36
+
+@onready var _container: Node3D = $FloorButtons
+
+func _ready() -> void:
+	var spacing = (Y_TOP - Y_BOTTOM) / float(BUTTON_COUNT - 1)
+	for i in range(BUTTON_COUNT):
+		var btn = BUTTON_SCENE.instantiate()
+		btn.name = "%02d" % i
+		btn.position = Vector3(0, Y_BOTTOM + i * spacing, 0)
+		_container.add_child(btn)
+		var sprite: Sprite3D = btn.get_node("ButtonSprite")
+		sprite.frame = i