Add sandbox stage builder scripts and functionality

- Introduced StageGizmos for hover highlighting, selection outlines, and rotation handles in the sandbox stage builder.
- Added StageGrid for an optional world-space grid overlay that adjusts with camera panning and zooming.
- Implemented StageSelection for geometric hit-testing and selection management of nodes in the sandbox.
- Created StageSpawner as a registry-driven factory for spawning terrain, props, and stickmen, allowing for dynamic template management.
- Each script includes necessary constants, state management, and public API methods for interaction.

scripts/stage_grid.gd

@@ -0,0 +1,46 @@
+class_name StageGrid
+extends Node2D
+## StageGrid - Optional world-space grid overlay for the Sandbox Stage Builder.
+##
+## Draws grid lines that pan/zoom with the camera, with a heavier major line
+## every few cells. Pure drawing; no hit-testing. Rendered behind the World by
+## keeping this node as the first child of the stage root.
+
+var camera: Camera2D = null
+var grid_size: float = 15.0
+var enabled: bool = true
+
+const GRID_COLOR := Color(1.0, 1.0, 1.0, 0.08)
+const MAJOR_COLOR := Color(1.0, 1.0, 1.0, 0.16)
+const MAJOR_EVERY := 5
+
+func _process(_delta: float) -> void:
+	if enabled:
+		queue_redraw()
+
+
+func _draw() -> void:
+	if not enabled or grid_size <= 0.0:
+		return
+	if camera == null or not is_instance_valid(camera):
+		return
+	var viewport_size := get_viewport_rect().size
+	var zoom := maxf(camera.zoom.x, 0.0001)
+	var half := viewport_size * 0.5 / zoom
+	var center := camera.get_screen_center_position()
+	var rect := Rect2(center - half, half * 2.0)
+	var line_width := 1.0 / zoom
+
+	var x := floorf(rect.position.x / grid_size) * grid_size
+	while x <= rect.end.x:
+		var grid_index := int(round(x / grid_size))
+		var color := MAJOR_COLOR if grid_index % MAJOR_EVERY == 0 else GRID_COLOR
+		draw_line(Vector2(x, rect.position.y), Vector2(x, rect.end.y), color, line_width)
+		x += grid_size
+
+	var y := floorf(rect.position.y / grid_size) * grid_size
+	while y <= rect.end.y:
+		var grid_index := int(round(y / grid_size))
+		var color := MAJOR_COLOR if grid_index % MAJOR_EVERY == 0 else GRID_COLOR
+		draw_line(Vector2(rect.position.x, y), Vector2(rect.end.x, y), color, line_width)
+		y += grid_size