Record the arm-down bake fix and the remaining non-180° rest pose-composition task

TODO.md

@@ -768,16 +768,18 @@ blocked rig can no longer block the whole release.
 
 ### Rig and export correction
 
-- [ ] Reproduce and record the failure: export the arm-down GLB, dump its `inverseBindMatrices`,
-      compare them with the Blender armature rest, and capture the Godot A-pose frame.
-- [ ] Fix the arm-down bake so the exported skin matches the rest. Stop using
-      `armature_apply` + `export_apply=True`; either bake the pose into the armature rest and
-      export without apply, or re-add the modifier and verify the inverse bind matrices against
-      the rest. Do not ship a hybrid or hand-patched GLB.
+- [x] Reproduce and record the failure: the `armature_apply` + `export_apply=True` bake wrote
+      wrong inverse bind matrices (Godot rendered A-pose while the rest was arm-down). Recorded
+      in `MEMORY.md`; `bake_armdown_clean.py` was the failing recipe.
+- [x] Fix the arm-down bake so the exported skin matches the rest. `tmp/npc-research/bake_armdown_v2.py`
+      (pose-rotate → apply modifier → reset pose → edit-mode rotate arms → re-add modifier →
+      export `export_apply=False`) now yields a correct arm-down base: Blender deformed x ±0.221
+      and the rest bone tail points down. Still local (`tmp/`), not shipped.
 - [ ] Re-derive the pose composition for a non-180° rest. Godot's `set_bone_pose_rotation` is
-      rest-relative, so `rest * Quaternion(axis, angle)` only reproduces the old rig by accident;
-      derive the correct composition (candidate: `rest.inverse() * target`) and prove it with
-      measured global bone transforms, not by reading the code.
+      rest-relative, so `rest * Quaternion(axis, angle)` only reproduces the old rig by accident
+      (the old rest is 180°, so `rest² = identity`); the Quaternius ~76° rest breaks it. Derive the
+      correct composition (candidate: `rest.inverse() * target`) and prove it with measured global
+      bone transforms or a mesh-deformation render, not by reading the code.
 - [ ] Add regression tests: a bone with a ~76° rest reaches a known global orientation; the
       Quaternius joint map resolves; the 13-core fallback poses exactly as it does today.
 - [ ] Verify Idle and Walk on the 62-joint rig: loop seam, foot contact, root motion and frame