AI: libigl bounded biharmonic skinning weights

[fernandotonon]

Parent epic: #397

Goal

Compute high-quality skinning weights for a mesh + skeleton using libigl's bounded biharmonic weights — no ML, header-only C++.

Scope

• Add libigl as a header-only dep (only the subset we need: igl::bbw, igl::boundary_conditions).
• CLI: qtmesh skin model.fbx --algo biharmonic -o skinned.fbx.
• GUI: Animation Mode action "Compute skin weights" when an entity has a skeleton but missing/poor weights.
• MCP: compute_skin_weights tool.
• Pair with the auto-rigger task (Epic: AI — Local-AI-assisted 3D workflows #397 Tier 3) — biharmonic weights are the natural completion step after Pinocchio places the skeleton.
• Preview: re-render the entity with new weights and a default test pose.

Acceptance Criteria

• A static mesh + a manually placed skeleton produces deformable skinning that follows bone rotations smoothly.
• Weights sum to 1.0 per vertex and are non-negative.
• Works for at least the standard humanoid skeleton from the test suite.
• Unit test on a simple mesh+skeleton fixture.
• Sentry breadcrumb ai.assist.skin_weights recorded.

Effort

~5 days.