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.

scenes/sandbox_stage.tscn

@@ -0,0 +1,12 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://scripts/sandbox_stage.gd" id="1_stage"]
+
+[node name="SandboxStage" type="Node2D"]
+script = ExtResource("1_stage")
+
+[node name="Camera2D" type="Camera2D" parent="."]
+position = Vector2(0, -400)
+zoom = Vector2(0.5, 0.5)
+
+[node name="World" type="Node2D" parent="."]