VertexPaint: Slice B — Paint-aware shading-mode authoring (UI + brush behaviour)

[fernandotonon]

Parent epic: #631
Depends on: Slice A (#632).

Goal

Expose the shading-mode data model to the user. Add the per-face mode picker, the "Paint as Flat / Paint as Smooth" stroke override, and the visual indicator that distinguishes flat from smooth faces while editing.

Scope

• VertexPaintController singleton (src/VertexPaintController.{h,cpp}) — new home for the authoring API:
  • setStrokeShadingMode(FaceShadingMode | Inherit) — the stroke-level override.
  • setFaceShadingModes(faceIndices, mode) — bulk operation.
  • getMeshShadingSummary(mesh) — for UI display ("12 flat faces, 234 smooth faces").
• Per-face mode picker in the inspector:
  • With a face (or set of faces) selected, a new "Shading" panel offers four buttons: Smooth / Flat / Textured / Textured Smooth.
  • Disabled (greyed) modes when the face has no material with a texture (Textured / Textured Smooth need a base map; the picker offers them anyway with a tooltip explaining the prereq).
  • Multi-select aware: setting a mode applies to all selected faces in one undo step.
• Stroke-level override in the brush toolbar:
  • Three buttons: "Paint as Flat" / "Paint as Smooth" / "Inherit face mode" (default).
  • "Inherit face mode" means: a stroke that touches a flat face writes its single colour; a stroke that touches a smooth face writes per-corner colours. The user gets predictable results when painting on a mixed mesh.
  • "Paint as Flat" forces every touched face into Flat mode and sets its colour to the brush colour. Useful for blocking-in flat-shaded sections fast.
  • "Paint as Smooth" forces touched faces into Smooth mode and writes per-corner colours. Useful for converting an imported flat-shaded mesh into a smoother look.
• Visual indicator:
  • In Edit Mode with face-selection active, flat faces display with a small dot-grid overlay at the face centre so the user can tell at a glance which faces are flat-shaded.
  • Toggle: "View → Show face shading mode" (default ON in Edit Mode).
• Undo:
  • SetFaceShadingModesCommand (the bulk version of VertexPaint: Slice A — Per-face shading mode (Flat / Smooth) with vertex-split #632's command) for the picker.
  • A stroke that changes face modes via "Paint as Flat" combines the mode change + the colour change into a single undoable stroke.
• Status bar / inspector summary: shows the per-mesh shading summary ("Smooth: 234 / Flat: 12 / Textured: 0") so users can verify their work.

Acceptance Criteria

• Inspector picker switches selected faces between modes correctly.
• Stroke-level "Paint as Flat" converts touched smooth faces to Flat and writes the brush colour as the face's single colour.
• Stroke-level "Paint as Smooth" converts touched flat faces to Smooth and writes per-corner colours.
• "Inherit face mode" behaves predictably on a mixed mesh.
• Visual flat-face indicator renders correctly when Edit Mode face selection is active.
• Multi-select bulk mode change works in one undo step.
• Per-mesh summary updates live as modes change.
• No regression in existing vertex paint (default behaviour with no stroke override matches today's behaviour on all-smooth meshes).
• Sentry breadcrumbs paint.vertex.shading_mode and paint.vertex.stroke_override per action.
• Headless-CI tests for each stroke-override mode on fixture meshes.

Effort

~6 days.