Narrow the Godot skinning blocker to the importer's rotated bind_pose

TODO.md

@@ -780,11 +780,13 @@ blocked rig can no longer block the whole release.
       (the old rest is 180°, so `rest² = identity`); the Quaternius ~76° rest breaks it. Fixed in
       commit `8b1bf73`: `rest * Quaternion(rest.inverse() * axis, angle)` — backward-compatible
       (the old 180° rest maps the axis to itself). Godot 277/277.
-- [ ] Godot skinning import: the arm-down GLB is now provably correct (raw vertices x ±0.175,
-      rest tail down, inverse bind matrices arm-down; Blender renders arm-down), but Godot 4.7.2
-      still renders the mesh in a T-pose. This is a Godot glTF importer skinning issue, not a
-      Blender bake or pose-formula issue. Next: isolate with a minimal 2-bone glTF, check the
-      Godot issue tracker, and try alternate export settings.
+- [ ] Godot skinning import: the arm-down GLB is provably correct (raw vertices x ±0.175, rest
+      tail down, inverse bind matrices arm-down; Blender renders arm-down), and the Godot-imported
+      mesh AABB is also arm-down (x ±0.221), but the render is a T-pose. Root cause narrowed to the
+      Godot glTF importer's `Skin.bind_pose` (inverse bind matrices): its bone direction is rotated
+      (Root `basis.y = −Z` instead of +Y), so the skin deforms the arm-down bind to T-pose. Same
+      result with `export_yup` True and False. Next: compare the old working `npc_casual.glb`
+      bind_pose, isolate with a minimal 2-bone glTF, and check the Godot 4.7.2 issue tracker.
 - [ ] 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