Character collision: generate capsules per vertex/weight group

[Weisl]

Summary

Add a way to generate capsule colliders for rigged characters driven by vertex/weight groups (e.g. one capsule per bone's weight group), rather than only the existing whole-object/selection-based capsule generation (collider_shapes/add_bounding_capsule.py).

Motivation

Characters are typically already weight-painted per bone for skinning. Reusing those weight groups to auto-generate a capsule per group would give a quick per-limb/per-bone collision rig without manually selecting geometry per bone.

Rough idea

• For each vertex group (or a user-selected subset) on the mesh, isolate the weighted vertices and fit a capsule to that subset, similar to how the current capsule fit works on a full object/selection.
• Likely wants to reuse the armature/bone data (group name <-> bone name convention) to orient the capsule along the bone rather than purely from vertex distribution, though a vertex-distribution-only fit may be a reasonable first pass.

Open questions

• Naming/parenting convention for the generated capsules relative to bones (parent to bone vs. object, existing collider naming scheme in collider_conversion/regenerate_name.py).
• How to handle vertex groups with very low/zero total weight or that don't correspond to a bone.
• UI: batch-generate for all groups vs. per-group operator.