Paint v2: Slice J — CLI/MCP parity, docs, tests

[fernandotonon]

Parent epic: #543
Depends on: Slices A–I (functionally needs A, B, C, D, I minimum)

Goal

Bring Paint v2 to project conventions: CLI, MCP, breadcrumbs, tests, docs.

Scope

• CLI — new qtmesh paint subcommand:
  • qtmesh paint model.fbx --layer add --name <n> --channel BaseColor -o out.fbx
  • qtmesh paint model.fbx --layer list [--json]
  • qtmesh paint model.fbx --layer merge-down --layer <n> -o out.fbx
  • qtmesh paint model.fbx --layer flatten -o out.fbx
  • qtmesh paint model.fbx --bake --engine unity --resolution 2048 -o out/
  • qtmesh paint model.fbx --apply-stencil <image.png> --camera "x,y,z,target" --channel BaseColor -o out.fbx
  • qtmesh paint --list-stamps, --list-presets, --list-palettes
  • Live stroke authoring is not in CLI scope.
• MCP tools:
  • paint_add_layer, paint_delete_layer, paint_reorder_layer, paint_merge_down, paint_flatten
  • paint_set_active_layer, paint_set_active_channel, paint_set_brush_preset, paint_set_color, paint_set_gradient
  • paint_apply_decal, paint_apply_stencil
  • paint_bake_pbr with engine target
  • JSON-RPC, main thread, no BlockingQueuedConnection.
• Sentry breadcrumbs (consolidated): paint.brush.*, paint.layer.*, paint.channel.*, paint.symmetry, paint.stabilizer, paint.projection.*, paint.decal.*, paint.derived_map.*, paint.tablet, paint.preset.apply, paint.palette.apply, paint.bake.*. ui.action for menu/toolbar clicks.
• Tests:
  • BrushEngine_test.cpp — gradients (Slice A), stamps (B), pressure (H).
  • PaintLayerStack_test.cpp — blend modes + composite (C).
  • PaintChannelRouter_test.cpp — channel dispatch + normal-from-height (D).
  • SymmetryEngine_test.cpp — axis-mirror correctness (E).
  • ProjectionPainter_test.cpp — projection / decal commit (F).
  • DerivedMaps_test.cpp — cavity / curvature / AO bake (G).
  • All headless-CI safe.
• Docs:
  • Replace MVP "Paint" section in CLAUDE.md with v2 architecture: layer stack, brush engine, channel router, symmetry, projection, derived maps, bake-up.
  • Add qtmesh paint examples to the CLI block.
  • Document on-disk layout (<AppData>/paint/...).
  • User-facing changelog summarising v2 features.

Acceptance Criteria

• Every CLI subcommand listed works with --json where applicable.
• Every MCP tool works via stdio + HTTP.
• CLI ↔ GUI ↔ MCP parity for every scriptable paint op.
• Breadcrumbs emitted for every user-visible paint action.
• All listed test files exist and pass on Linux / macOS / Windows CI.
• CLAUDE.md Paint section rewritten and accurate.

Effort

~6 days.