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.

scripts/stickman_factory.gd

@@ -0,0 +1,47 @@
+class_name StickmanFactory
+extends RefCounted
+## StickmanFactory - Runtime factory for spawning rig instances from .stk files
+## (Phase 9).
+##
+## Loads a versioned .stk JSON dictionary and instantiates a master_rig.tscn,
+## adapting it via StkRigAdapter. Consumed by the standalone test harness, not
+## referenced by the editor.
+
+# ---------------------------------------------------------------------------
+# Constants
+# ---------------------------------------------------------------------------
+
+const RIG_SCENE := preload("res://master_rig.tscn")
+
+# ---------------------------------------------------------------------------
+# Public API
+# ---------------------------------------------------------------------------
+
+static func load_stk(path: String) -> Dictionary:
+	var file := FileAccess.open(path, FileAccess.READ)
+	if file == null:
+		push_warning("StickmanFactory: failed to open '%s' (error %d)." % [path, FileAccess.get_open_error()])
+		return {}
+
+	var text := file.get_as_text()
+	file.close()
+
+	var parsed: Variant = JSON.parse_string(text)
+	if parsed == null or not parsed is Dictionary:
+		push_warning("StickmanFactory: failed to parse '%s' as JSON." % path)
+		return {}
+
+	return parsed as Dictionary
+
+
+static func spawn_from_data(stk_data: Dictionary) -> Node2D:
+	var rig := RIG_SCENE.instantiate() as Node2D
+	StkRigAdapter.apply(stk_data, rig)
+	return rig
+
+
+static func spawn(path: String) -> Node2D:
+	var data := load_stk(path)
+	if data.is_empty():
+		return null
+	return spawn_from_data(data)