fix(weapons): the wrist turns the hand, and the gun stays on the aim line

The weapon is a child of a BoneAttachment3D on the trigger hand, so the two were
welded by construction. Every degree `wrist_r` turned swung the barrel the same
degree off the aim line — and took with it every control that could have
corrected for it, because they are all expressed relative to that same hand.
There was no combination of sliders that aligned a hand to a gun, which is the
one thing the knob exists for.

Both outcomes are now computed where the hand's local pose is set: the rotation
the hand would take without the wrist offset, and the one it takes with it. The
hand gets the second; the difference between them is exactly the counter-rotation
the weapon mount needs, in the hand's own local frame, and
`SkinnedPlayerModel._hold_weapon_still` applies it to the mount each frame. The
gun ends up precisely where the solver put it.

That also gives the two controls a clean split, which is what makes them usable
together:

  TRIGGER / SUPPORT WRIST (hold)     turns the HAND, gun stays on the aim line
  Grip roll / pitch / yaw (anchors)  turns the GUN inside the hand

Identity when `wrist_r` is untuned, so a character nobody has tuned mounts its
weapon exactly as before.

Applied in `_process` rather than inside the modifier pass on purpose. The gun's
mount is not something the skeleton owns, and the compensated value only changes
when a slider moves or the ADS blend travels, so one frame of lag is a fraction
of a degree; reaching into the modifier to touch a scene node would be worse.

wrist_gun_check asserts both halves, because only asserting the first is how
this shipped broken: the hand must TURN, or the knob does nothing, and the gun
must NOT, or the knob cannot be used. Across both poses and all three axes the
hand turns 28.2-28.7 degrees for a 0.5 rad knob and the gun moves 0.1-0.6 —
against the ~28 it would move if it were still following the wrist. The residue
is the arm's own IK settling, since the hand's rotation feeds the chain that
places the shoulder.

Co-Authored-By: Claude Opus 5 <[email protected]>

.claude/skills/character-pipeline/references/verification.md

@@ -47,6 +47,7 @@ Two related traps:
 | `anchor_shift_check.gd` | the hand anchors move in the GUN's frame, both poses | 0 failures |
 | `anchor_drag_check.gd` | dragging a marker writes the knob the mouse asked for | 0 failures |
 | `hold_pose_check.gd` | the lab shows only the selected pose's knobs; every wrist axis turns its hand | 0 failures |
+| `wrist_gun_check.gd` | the wrist turns the hand and NOT the gun welded to it | hand ~28°, gun < 1° |
 | `anim_capture.gd` / `orbit_capture.gd` | renders, for looking | — |
 | `roster_capture.gd` | one photo of every character, from the picker | — |
 | `ui_capture.gd` | one photo of every menu screen | — |