feat: Implement Stickman Factory and Test Harness for runtime rigging

- Added StickmanFactory to load and instantiate .stk files into master_rig.tscn.
- Created a Test Harness scene for debugging bone scales, vector drawing offsets, and IK limits.
- Updated stk_rig_adapter to fit head bone and mount head shapes as full geometry.
- Enhanced README and PROJECT documentation to reflect new features and usage.
- Introduced UI elements for file selection and visual debugging in the Test Harness.
- Refactored code to improve clarity and maintainability, including removal of unused functions.

scenes/test_harness.tscn

@@ -0,0 +1,12 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/test_harness.gd" id="1_harness"]
+
+[node name="TestHarness" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("1_harness")