Generate collision shapes from vertex group weights (rigged/character meshes)

[Weisl]

Context

Shape generation currently sources geometry from selected objects, edit-mode selections, or loose islands. There's no way to scope generation to a vertex group, which is the natural workflow for rigged/character meshes where a collision shape should follow a specific bone-weighted region rather than the whole mesh or a manual selection.

Proposal

Add an option to source the geometry used for shape-fitting (box/sphere/capsule/convex/etc.) from a vertex group - e.g. all vertices with weight above a threshold - feeding into the existing bounding-fit algorithms instead of the full mesh/selection.

Needs a design decision on UI placement (e.g. a vertex-group dropdown + weight threshold alongside the existing space/creation-mode controls) before implementation.

Notes

• Narrower audience than most existing features (character/rigged-prop pipelines specifically) - worth confirming priority against the rest of the backlog.
• No existing issue found covering this in the tracker as of this writing.

Acceptance criteria

• A vertex group can be selected as the geometry source for at least one shape type as a first pass (e.g. capsule, since it's the most common per-bone use case).
• Existing per-shape fitting logic is reused, not duplicated.
• UI clearly communicates when a vertex-group source is active vs. the default full-mesh/selection source.