Skel: Slice D — Skin weight painting

[fernandotonon]

Parent epic: #554
Depends on: Slice A (#555). Best-with: Paint v2 Slice C (#546).

Goal

Paint skin weights with a proper brush — radius, strength, falloff, symmetry, pressure — plus the standard weight-paint utility ops: normalize, mirror, smooth, lock per-bone. Today's BoneWeightOverlay is read-only; this slice makes it interactive.

Scope

• New singleton SkinWeightController (src/SkinWeightController.{h,cpp}).
• Brush integration:
  • Reuse Paint v2's BrushEngine (Epic: Paint v2 — Gradients, textured brushes, layers, full PBR painting #543 / Slice C Paint v2: Slice C — Layer stack (named, blend modes, opacity, masks, reorder) #546): radius / strength / falloff / pressure / symmetry come for free.
  • If Paint v2 Slice C hasn't shipped when this slice starts, prototype against TexturePaintController and migrate when it lands.
  • Brush colour value collapses to a scalar weight 0..1 (since each "stroke" targets a single active bone).
• Tools:
  • Paint (add to active bone's weight, ramping with strength).
  • Smear / blur (low-pass filter the weights under the brush).
  • Erase (paint zero — bone influence drops to zero, weight redistributed to remaining bones to keep normalized).
  • Fill connected (flood-fill a UV-connected island at the active bone, with falloff from the click point).
• Utility ops (no brush — applied to selection or whole mesh):
  • Normalize — every vertex's bone weights sum to 1.0.
  • Mirror weights — depends on Slice E's bone naming; mirror weights across an axis using _l/_r (or any configured naming) bone pairs.
  • Smooth weights — Laplacian smoothing over the mesh's vertex adjacency (via HalfEdgeMesh).
  • Lock bone — selected bones can't be modified by paint ops (their weights are frozen; other bones absorb redistributions).
  • Limit weights — clamp each vertex to at most N influences (default 4 — game-engine constraint).
• Live visualization: extends BoneWeightOverlay to show active bone weight under the cursor (number readout + heatmap toggle).
• Per-vertex weight inspector: select a vertex (Edit Mode integration), see its bone-weight list, edit a single value numerically.
• Undo: PaintWeightStrokeCommand (per-stroke, like Paint v2), WeightOpCommand (utility ops). Both mergeable for drags.

Acceptance Criteria

• Paint weights on a fixture skinned mesh and see immediate deformation change when posing.
• Normalize, mirror, smooth, lock all work on a fixture and produce expected weight distributions.
• Limit-weights (4 influences) preserves the largest weights and renormalizes.
• Per-vertex inspector shows live values and edits round-trip.
• Active-bone-weight overlay updates as the cursor moves.
• Mirror requires Slice E plumbing — fall back to "axis-position mirror" if Slice E hasn't landed.
• All operations undoable.
• Sentry breadcrumbs scene.skel.weight.*.
• Headless-CI tests cover normalize, mirror, smooth, lock, limit on a fixture.

Effort

~10 days, plus 2 buffer if BrushEngine integration needs prototype-then-migrate.