feat: Implement draggable torso and head IK targets in the test harness

- Added draggable handles for the torso and head to the test harness.
- Updated `IK_HANDLE_PATHS` to include new entries for "Head" and "Torso".
- Implemented distinct colors for the torso (magenta) and head (yellow) markers.
- Added a visual aid (aim line) to indicate the head's LookAt target direction.
- Ensured that dragging the torso moves only the torso marker, allowing for limb stretching towards stationary targets.

scripts/whole_stickman_preview.gd

@@ -271,6 +271,16 @@ func set_show_guide(enabled: bool) -> void:
 	preview_area.queue_redraw()
 
 
+## Returns the preview-space position of a guide joint (a master-space
+## GUIDE_JOINTS entry mapped through _guide_to_preview). Guarded against
+## unknown joint names.
+func get_guide_joint_preview(joint_name: String) -> Vector2:
+	if not GUIDE_JOINTS.has(joint_name):
+		push_warning("WholeStickmanPreview: unknown guide joint '%s'." % joint_name)
+		return Vector2.ZERO
+	return _guide_to_preview(GUIDE_JOINTS[joint_name])
+
+
 func reset_view() -> void:
 	_zoom = 1.0
 	_pan_offset = Vector2.ZERO