Paint v2: Slice G — Cavity / curvature / AO masks

[fernandotonon]

Parent epic: #543
Depends on: Slice C (#546)

Goal

Auto-generate per-mesh cavity, curvature, and AO maps; use them as brush colour sources or layer masks — the canonical way to add edge wear, crevice dirt, and weathering.

Scope

• Generators:
  • Cavity — concave dark; per-vertex from normal angle to neighbours (via HalfEdgeMesh), then rasterised to UV space with seam dilation (reuse VertexColorBaker seam plumbing).
  • Curvature — signed cavity (concave -, convex +). Edge wear from positive curvature.
  • AO — short-ray hemispherical occlusion. CPU bake, few seconds on 5k-tri meshes.
• Cache in <AppData>/paint/derived_maps/<mesh-hash>/. Invalidate on topology change via EditableMesh revision counter.
• Usage:
  • Brush colour source: brush colour × derived map value at painted UV; combines with gradient/stamp colour.
  • Layer mask: a layer's mask initialised from a derived map; user paints on top to refine.
• One-click recipes:
  • "Add edge wear" — inverse-curvature-masked layer, default = light bare metal.
  • "Add crevice dirt" — cavity-masked layer, default = dark grime.
  • "Add AO darken" — multiply layer of AO onto BaseColor.
• "Recalculate derived maps" button (manual invalidate).

Acceptance Criteria

• Cavity, curvature, AO compute correctly on a fixture with clear convex/concave regions.
• Maps cache + reload without recomputing.
• Each map can drive brush colour or initialise a layer mask.
• Each one-click recipe produces visibly correct results.
• Maps recompute on topology change.
• Sentry breadcrumb paint.derived_map.*.

Effort

~7 days.