feat(weapons): scale the gun to the arm, and close the hands around it

The hold was wrong on every character in three independent ways, all of
them a constant standing where a measurement belonged.

MOUNT. set_weapon seated the weapon with rotation_degrees = (0, 90, -90).
A bone attachment is expressed in the BONE's axes and no two rigs agree on
those, so one constant mounted the gun differently on every model. It never
needed to be right — the pose layer aims by rotating the wrist until the
weapon's forward lies on the aim line, so the identity means "forward is
the hand bone's -Z", true on any rig, and the wrist absorbs the roll.

SIZE. The set is modelled at real-world scale; an M4 is 0.84 m butt to
muzzle and these characters have 0.47 m arms against an adult 0.52. That
put the handguard 0.66 m from the support shoulder, 0.2 m past reach, so
the support hand was slid back down the weapon until it fitted — on Taila
from an authored 0.35 m to 0.083 m, which puts both fists together at the
grip. Two hands on a pistol, not a rifle.

Fixed by solving the support arm's triangle rather than picking a factor:
its hand must reach stock+fore ahead of the pocket from a shoulder half a
shoulder-width off the axis, so scale the gun to the largest that keeps the
handguard inside that reach. Taila and Kiyoko now come out at their own
scales (0.217 and 0.213 m of hand separation) with the support hand at its
FULL authored handguard distance and the slide-back loop never firing. The
loop also has a floor now: a straight support arm beats no handguard hold.

FINGERS. Nothing posed them — every hand was flat and open, which is the
loudest possible tell that a character is not holding anything. They close
now, about an axis derived from each hand's own anatomy in the rest pose:
along = wrist to middle knuckle, palm = middle knuckle to thumb tip (the
thumb opposes the fingers, so it marks the palm side by construction), curl
= along x palm. The trigger finger gets a much shallower curl than the
rest, because it lies along the trigger.

Finger bones resolve by ROLE across all three naming families met so far —
Rigify DEF-f_index.01.L, VRoid J_Bip_L_Index1, Blender IndexFinger1_L —
ordered by depth below the hand rather than by the number in the name,
which is not consistent between them.

Verified by render on Taila and by measurement on Kiyoko: stock at the
shoulder, trigger hand on the grip, support hand out on the handguard,
fingers wrapped, arms not crossing. Smoke 0 failures.

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

assets/characters/skins/kiyoko.rig.json

@@ -13,20 +13,72 @@
     ],
     "shin.R": "J_Bip_R_LowerLeg",
     "shin.L": "J_Bip_L_LowerLeg",
-    "thigh.R": "J_Bip_R_UpperLeg",
     "thigh.L": "J_Bip_L_UpperLeg",
-    "shoulder.L": "J_Bip_L_Shoulder",
+    "thigh.R": "J_Bip_R_UpperLeg",
     "shoulder.R": "J_Bip_R_Shoulder",
-    "forearm.R": "J_Bip_R_LowerArm",
+    "shoulder.L": "J_Bip_L_Shoulder",
     "forearm.L": "J_Bip_L_LowerArm",
-    "upper_arm.L": "J_Bip_L_UpperArm",
+    "forearm.R": "J_Bip_R_LowerArm",
     "upper_arm.R": "J_Bip_R_UpperArm",
-    "toe.L": "J_Bip_L_ToeBase",
+    "upper_arm.L": "J_Bip_L_UpperArm",
     "toe.R": "J_Bip_R_ToeBase",
+    "toe.L": "J_Bip_L_ToeBase",
     "foot.L": "J_Bip_L_Foot",
     "foot.R": "J_Bip_R_Foot",
-    "hand.L": "J_Bip_L_Hand",
-    "hand.R": "J_Bip_R_Hand"
+    "hand.R": "J_Bip_R_Hand",
+    "hand.L": "J_Bip_L_Hand"
+  },
+  "fingers": {
+    "thumb.L": [
+      "J_Bip_L_Thumb1",
+      "J_Bip_L_Thumb2",
+      "J_Bip_L_Thumb3"
+    ],
+    "index.L": [
+      "J_Bip_L_Index1",
+      "J_Bip_L_Index2",
+      "J_Bip_L_Index3"
+    ],
+    "middle.L": [
+      "J_Bip_L_Middle1",
+      "J_Bip_L_Middle2",
+      "J_Bip_L_Middle3"
+    ],
+    "ring.L": [
+      "J_Bip_L_Ring1",
+      "J_Bip_L_Ring2",
+      "J_Bip_L_Ring3"
+    ],
+    "pinky.L": [
+      "J_Bip_L_Little1",
+      "J_Bip_L_Little2",
+      "J_Bip_L_Little3"
+    ],
+    "thumb.R": [
+      "J_Bip_R_Thumb1",
+      "J_Bip_R_Thumb2",
+      "J_Bip_R_Thumb3"
+    ],
+    "index.R": [
+      "J_Bip_R_Index1",
+      "J_Bip_R_Index2",
+      "J_Bip_R_Index3"
+    ],
+    "middle.R": [
+      "J_Bip_R_Middle1",
+      "J_Bip_R_Middle2",
+      "J_Bip_R_Middle3"
+    ],
+    "ring.R": [
+      "J_Bip_R_Ring1",
+      "J_Bip_R_Ring2",
+      "J_Bip_R_Ring3"
+    ],
+    "pinky.R": [
+      "J_Bip_R_Little1",
+      "J_Bip_R_Little2",
+      "J_Bip_R_Little3"
+    ]
   },
   "chains": [
     {