fix: rebuild the elbow as a readable hinge in vertex-weight-limit

[TMHSDigital]

Summary

Round three on the vertex-weight-limit hero. The previous revision still read as a bent tube with a teal donut at the kink: the bellows swallowed the whole joint, the cheeks were hidden behind it, and the hinge cap tilted with the forearm — because the pin and caps were skinned to Elbow, so the -35° pose tilted them off the cheeks.

The hinge is now built like a hinge:

• Clevis cheek plates visible (orange, z 1.05–1.55, flanking the joint)
• Pin + bolt caps skinned to Shoulder — the pin stays with the clevis; caps sit flush as bolt heads and cannot tilt
• Knuckle barrel skinned to Elbow — the forearm rotates around the pin
• Flex zone slimmed to a ribbed cuff behind the hinge (still the five-influence pruned zone, teal-accented, bright seal hoop on it)
• Fingers chunked so the gripper reads at distance

Contract evidence: every cuff ring sits inside the five-bump z window, so pre_max=5 holds; measured values identical on .scratch/blender-5.1.2-windows-x64/blender.exe (Blender 5.1.2) and .scratch/blender-4.5.11-windows-x64/blender.exe (Blender 4.5.11 LTS): verts=1840 pre=5 post=4 limited=160 sum_err=2.98e-08 pose_dev=2.76e-03 lbs_err=2.98e-07 root_move=0. Re-falsified: probe A exit 4, probe B exit 6 (sums 1.616e-02), probe C exit 8 (LBS dev 1.779). No tolerance loosened. 4.5 render path green.

Full-size crops of the elbow hinge and claw inspected at native resolution. Group contact sheet refreshed (luminance 111.8, in set range). Gallery rebuilt, detail HTML re-read.

Test plan

• Validate, manifest, counts
• Blender 5.1 + 4.5 smoke (vertex-weight-limit step)

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