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/test_harness.gd

@@ -24,6 +24,8 @@ const BONE_COLOR_RIGHT := Color(1.00, 0.50, 0.20)
 const BONE_COLOR_CENTRAL := Color(1.00, 1.00, 1.00)
 const HANDLE_COLOR_HAND := Color(0.00, 1.00, 0.00)   # green
 const HANDLE_COLOR_FOOT := Color(0.00, 0.50, 1.00)   # blue
+const HANDLE_COLOR_HEAD := Color(1.00, 1.00, 0.00)   # yellow
+const HANDLE_COLOR_TORSO := Color(1.00, 0.00, 1.00)  # magenta
 
 const SKELETON_PATH := "Skeleton2D"
 
@@ -35,11 +37,28 @@ const QUICK_LOADS: Array = [
 ]
 
 ## IK handle node paths (relative to rig root), keyed by handle name.
+## The four limb targets drive TwoBoneIK; "Head" is the LookAt aim point and
+## "Torso" is the hip anchor (dragging it translates the whole rig via its
+## RemoteTransform2D — no target-following logic).
 const IK_HANDLE_PATHS: Dictionary = {
 	"Left_Hand":  "IK_Targets/Left_Hand",
 	"Right_Hand": "IK_Targets/Right_Hand",
 	"Left_Leg":   "IK_Targets/Left_Leg",
 	"Right_Leg":  "IK_Targets/Right_Leg",
+	"Head":       "IK_Targets/Head",
+	"Torso":      "IK_Targets/Torso",
+}
+
+## Leaf bone → IK target node path (relative to rig root), keyed by bone name.
+## Used by the bone overlay to draw the forearm/shin segments out to their
+## wrist/ankle targets. The Head leaf is NOT listed here — its IK target is a
+## LookAt aim point, so it draws along its own bone direction instead (see
+## _draw_bones' leaf fallback).
+const LEAF_BONE_IK_PATHS: Dictionary = {
+	"LeftLowerArm":  "IK_Targets/Left_Hand",
+	"RightLowerArm": "IK_Targets/Right_Hand",
+	"LeftLowerLeg":  "IK_Targets/Left_Leg",
+	"RightLowerLeg": "IK_Targets/Right_Leg",
 }
 
 # ---------------------------------------------------------------------------
@@ -282,9 +301,35 @@ func _draw_bones() -> void:
 		var origin := bone.global_position
 		_debug_overlay.draw_circle(origin, dot_r, color)
 
-		var parent := bone.get_parent()
-		if parent is Bone2D:
-			_debug_overlay.draw_line((parent as Bone2D).global_position, origin, color, line_w)
+		# Collect the bone's Bone2D children (true bone segments, including the
+		# nested lower bones).
+		var bone_children: Array[Bone2D] = []
+		for child in bone.get_children():
+			if child is Bone2D:
+				bone_children.append(child as Bone2D)
+
+		if bone_children.is_empty():
+			# Leaf bone — draw out to its IK target (forearm/shin), falling
+			# back to the bone's own direction (bone_angle + rotation) when
+			# there is no target (e.g. the Head, whose target is a LookAt aim
+			# point, not a joint).
+			var target := _leaf_bone_target(bone)
+			if target != null:
+				_debug_overlay.draw_line(origin, target.global_position, color, line_w)
+			else:
+				var fallback := origin + Vector2(bone.get_length(), 0.0).rotated(deg_to_rad(bone.bone_angle)).rotated(bone.global_rotation)
+				_debug_overlay.draw_line(origin, fallback, color, line_w)
+		else:
+			for child_bone in bone_children:
+				_debug_overlay.draw_line(origin, child_bone.global_position, color, line_w)
+
+
+func _leaf_bone_target(bone: Bone2D) -> Node2D:
+	if not LEAF_BONE_IK_PATHS.has(bone.name):
+		return null
+	if _rig == null or not is_instance_valid(_rig):
+		return null
+	return _rig.get_node_or_null(NodePath(str(LEAF_BONE_IK_PATHS[bone.name]))) as Node2D
 
 
 func _draw_ik_handles() -> void:
@@ -296,11 +341,34 @@ func _draw_ik_handles() -> void:
 		var handle := _ik_handles[handle_name] as Marker2D
 		if handle == null or not is_instance_valid(handle):
 			continue
-		var color := HANDLE_COLOR_HAND if handle_name.ends_with("Hand") else HANDLE_COLOR_FOOT
+		var color := _handle_color(handle_name)
 		var pos := handle.global_position
 		_debug_overlay.draw_circle(pos, r, color)
 		_debug_overlay.draw_arc(pos, r, 0.0, TAU, 24, Color(1.0, 1.0, 1.0, 0.8), outline_w)
 
+	# Head aim line — visual aid showing what the head bone is aiming at.
+	var head_handle: Marker2D = _ik_handles.get("Head", null) as Marker2D
+	if head_handle != null and is_instance_valid(head_handle) \
+			and _skeleton != null and is_instance_valid(_skeleton):
+		var head_bone := _skeleton.get_node_or_null(NodePath("Torso/Head")) as Bone2D
+		if head_bone != null and is_instance_valid(head_bone):
+			_debug_overlay.draw_line(
+				head_bone.global_position,
+				head_handle.global_position,
+				Color(1.0, 1.0, 0.0, 0.5),
+				1.5 / zoom
+			)
+
+
+func _handle_color(handle_name: String) -> Color:
+	match handle_name:
+		"Head":
+			return HANDLE_COLOR_HEAD
+		"Torso":
+			return HANDLE_COLOR_TORSO
+		_:
+			return HANDLE_COLOR_HAND if handle_name.ends_with("Hand") else HANDLE_COLOR_FOOT
+
 
 func _bone_color(bone_name: String) -> Color:
 	if bone_name.begins_with("Left"):