Paint v2: Slice B — Textured / stamp brushes

[fernandotonon]

Parent epic: #543

Goal

Paint with an image as the brush — per-stamp images (a leaf, a scratch, a stencil shape) and tiling source textures (wood grain, brick, fabric).

Scope

• Extend BrushEngine's footprint from { round | square } to { round | square | stampImage | tilingSource }.
• Stamp image mode:
  • PNG/TGA with alpha; greyscale = mask.
  • Each stamp paints one instance scaled to brush radius, with brush colour or gradient (Slice A) modulating it.
  • Rotation: none | fixed angle | stroke direction | random jitter.
  • Spacing — distance between stamps along the stroke as % of radius.
  • Scatter — random perpendicular offset.
  • Size jitter, opacity jitter per stamp.
• Tiling source mode:
  • Tileable image sampled under the footprint in UV-space.
  • Useful for wood grain, brick, fabric where the pattern is in the image, not the footprint.
  • Tiling scale, rotation, offset.
• Bundled (CC0): 6 stamps (soft circle, hard circle, charcoal, spatter, scratch lines, foliage cluster); 4 tilings (wood, brick, fabric, concrete).
• Stamp library panel in brush inspector: thumbnail grid, drag-and-drop import, delete, rename. Storage in <AppData>/paint/stamps/ and <AppData>/paint/tilings/.
• Cache: stamp images pre-rasterised once per brush-relative footprint.

Acceptance Criteria

• Stamp brushes produce visibly correct stamps with rotation, spacing, scatter, jitter.
• Tiling source brush tiles a wood-grain texture across a stroke without visible seams.
• User can import a custom PNG as a stamp.
• All 10 bundled assets ship across macOS, Windows, Linux.
• Gradient brush (Slice A) + textured stamp combine correctly.
• Strokes undoable.
• Sentry breadcrumb paint.brush.stamp with stamp name + mode.
• Headless-CI tests cover stamp footprint computation and tiling sampling.

Effort

~8 days.