Improve rig fitting, helper controls, and weight smoothing

[Princeamor]

Summary

This pull request improves the rig fitting workflow for manually positioned skeletons and softens several problem areas in generated skin weights.

The main functional changes are:

• added mesh-drag vertex snapping with a user-controlled snap strength slider
• added condensed main-chain visibility toggles in Position Joints mode
• centered pelvis, spine, chest, neck, and head placement against the mesh instead of the front surface
• reduced finger joint helper circle size and made helper visibility respect hidden chains
• prevented branching parent bones such as chest and pelvis from being auto-rotated when child bones are moved independently
• expanded weight smoothing around pelvis, thighs, buttocks, shoulders, clavicles, neck, and upper arms
• added focused regression tests for the new placement, chain grouping, rotation, and smoothing behavior

Why

These changes target the issues seen during rig fitting and auto-weighting on humanoid avatars:

• joint placement needed stronger assistance when dragging bones over the mesh
• main chains needed to be easier to isolate visually during joint positioning
• centerline bones were drifting too far toward the front of the character
• shoulder and hip areas needed softer, more natural blending similar to the imported DAE reference
• chest and shoulder orientation could drift when moving branching child bones

What changed

Position Joints workflow

• Added a snap strength slider with a 0 to 20 range for mesh-drag snapping.
• Added visible-chain checkboxes for condensed main chains, with fingers grouped into the hand chain.
• Wired helper visibility changes so hidden chains are removed from the active helper display.

Bone placement and orientation

• Added nearest-vertex snap blending during mesh drag.
• Added mesh-centerline targeting for primary centerline bones.
• Applied automatic centerline snapping after skeleton load for pelvis, spine, chest, neck, and head chains.
• Preserved template orientation on branching parent bones during independent child movement.

Weight smoothing

• Expanded boundary smoothing to keep more multi-bone influence at seam regions.
• Added along-bone gradients so bone centers stay dominant while joints retain blended influence.
• Added stronger symmetric socket smoothing for:
  • pelvis to thigh
  • spine/chest/neck to shoulder chain
  • clavicle to upper arm
• Added pelvis-basin smoothing to improve the glute and central pelvis region.
• Strengthened torso-to-upperarm blending in the front shoulder area.

Helper and utility updates

• Added condensed chain grouping utilities for visibility and labeling.
• Added smaller helper points for finger joints.
• Ensured hidden chain roots affect helper rendering consistently.

Investigation support

• Added temporary scripts under tmp/ for comparing skeleton topology and weight profiles during debugging.

Testing

• npm run build
• Focused regression tests were added for:
  • chain grouping utilities
  • mesh drag snapping and centerline helpers
  • independent branching-bone movement behavior
  • advanced weight smoothing boundaries

Notes for review

• This PR includes temporary analysis scripts in tmp/ because they were used to validate skeleton and weight-profile differences during the investigation.
• The changes are concentrated in the rig-editing and skin-weight smoothing flow rather than import/export behavior.

[scottpetrovic]

Can you create github issues for each of these so we can have a discussion first. There is too much going on with this ticket, and I am not sure if a lot of this code is even necessary.

[Princeamor]

Done

[scottpetrovic]

I pulled down the code and looked to see how the UX/features have changed. In the current state this isn't something I would be able to ever merge. This is what I see when I load one of the models.

We can leave this open for reference, but I think we will need more discussion on the "Issues" area before worrying about any coding. Another thing with PRs is it is good to focus on only one specific thing. Unless you are the main developer, it usually is best to just do one thing at a time. It will improve the chance something will get merged in the long run.

[Princeamor]

I have not done a lot in groups. Thank you for tolerating it.