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.

docs/phase9_round4_bugfix_spec.md

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+# Phase 9 Round 4 — Bugfix: Head Position vs. the Editor's Silhouette Guide
+
+## Overview
+
+User report: the head is **too high** in the harness. Reference: the editor's silhouette guide
+head is a circle; the bottom of the mounted head shape must line up with the bottom of that
+circle relative to the torso bone, so the head **overlaps the torso** some.
+
+### Guide geometry (source of truth, `scripts/whole_stickman_preview.gd`)
+
+| Constant | Value |
+|---|---|
+| `GUIDE_JOINTS["Neck"]` (`:61`) | `(0, −391.5)` — the Head bone origin |
+| `GUIDE_JOINTS["Head"]` (`:62`) | `(0, −463.5)` — the head circle **center** |
+| `GUIDE_HEAD_RADIUS` (`:49`) | `100.0` |
+
+→ The guide's head circle bottom = `−463.5 + 100 = −363.5` — **28 px below the neck**,
+overlapping the torso's top region (torso spans `0 → −391.5`) by 28 px.
+
+### Current behavior (wrong)
+
+Round 3 zeroes the Head driver's local position, so `Body/Head` sits on the Head bone origin
+(the neck, `y = −391.5`); the mounted chin is the local origin → the chin lands at the neck
+and the head floats above the torso with no overlap.
+
+### Required behavior
+
+The mounted head's bottom (chin) must land at `y = −363.5` — i.e. the mounted head geometry
+gets a `+28 px` local Y offset so the chin sits 28 px below the neck joint, matching the
+guide circle's bottom and overlapping the torso.
+
+## Fix specification — `scripts/stk_rig_adapter.gd`
+
+1. Add `const HEAD_CHIN_DROP := 28.0` with a doc comment deriving the value
+   (guide head circle bottom `−363.5` − neck `−391.5` = 28; the guide circle has radius 100
+   centered at the Head joint `(0, −463.5)`).
+2. In the head branch of the mount (where `θ = 0`, `s = 1`), after transforming the points,
+   translate them by `Vector2(0.0, HEAD_CHIN_DROP)` — the chin (local origin) ends up 28 px
+   below the node origin (the neck).
+3. The flip anchor rule from Round 3 is unchanged (a flipped head still attaches at its cap
+   top; the drop applies identically).
+4. Nothing else changes (no driver/overlay/serialization changes).
+
+Note: the offset is applied **after** the part scale/rotation composition (it is a rig-space
+fixture position, not part geometry), so a 180°-rotated head drops by the same 28 px.
+
+## Files modified
+
+| File | Changes |
+|---|---|
+| `scripts/stk_rig_adapter.gd` | `HEAD_CHIN_DROP` const + head-points translation. |
+| `docs/phase9_round4_bugfix_spec.md` | This file. |
+
+## Test plan
+
+1. Parse check: `..\Godot_v4.7.1-stable_win64_console.exe . --headless --check-only --quit`.
+2. Headless smoke test (spawn `break.stk`, stack enabled, 2 frames):
+   - `Body/Head` world position ≈ `(0, −391.5)` (node still at the neck).
+   - Mounted head chin point ≈ local `(0, +28)`; world chin ≈ `(0, −363.5)`.
+   - Head top ≈ world `(0, −363.5 − 219)` ≈ `(0, −582)`; the head now overlaps the torso's
+     top region (torso top at −391.5) by ≈ 28 px.
+   - Regression: the head's other Round 3 properties (scale ≈160 face, one node per shape,
+     IK following, flip behavior) unchanged.
+3. Manual harness check (F6): head bottom aligned with the guide circle's bottom; head
+   overlaps the torso.
+
+## Design decisions
+
+| # | Decision | Justification |
+|---|---|---|
+| D1 | Fixed `+28 px` rig-space offset (not relative to the head's drawn size) | The guide circle is a fixed rig fixture; the requirement is to align to its bottom. |
+| D2 | Offset applied to the mounted points (not the driver) | The Head driver's zeroing stays (rotation about the neck with the LookAt); a driver offset would swing with the bone rotation. |