[crisprintsstuff]

Adds a Paint Groups brush (requested by the maintainer): paint face groups directly instead of masking and then grouping.

Behaviour

• In the brush shelf after Paint (no number key; the first ten are unchanged).
• Each stroke makes a new group. Alt+stroke grows the group under the start of the stroke.
• Paints whole triangles whose centre is inside the brush.
  • Follows symmetry, including radial, and Front faces only.
  • Leaves hidden faces alone and never moves the surface.
  • Fills dabs along the path, so fast strokes leave no gaps.
• Turns the group colors on when you start painting, and says so in the status bar.
• One undo step per stroke (a face edit). Escape mid-stroke puts the groups back.
• The brush panel shows only what applies: Radius, Size mode, Pen pressure sets radius, and Front faces only.
• Agents get a clear error for this brush instead of a stroke that does nothing.

How

• Mesh::paint_group visits only chunks whose bounds meet the brush sphere.
• It bumps the face version without recomputing per-vertex visibility, which stays valid because only groups change.
• The stroke is app-driven (Drag::GroupPaint, crates/oxy-app/src/group_paint.rs), like Tube, not a dab kernel.
• BrushKind::PaintGroups is added everywhere brush kinds are listed: preset and settings keys, icon (amber tint), description, and gravity exclusion.

Performance

Per dab on a 1,310,720-triangle sphere (release build, 4 cores, from a throwaway timing loop not in this PR):

Radius	Time per dab
0.05	0.07 ms
0.2	0.21 ms

Sculpting brushes are untouched.

Verified

• cargo fmt --check, clippy -D warnings, cargo test --workspace, check_corpus_ip.py.
• New tests:
  • oxy-mesh: only the sphere is painted; front faces only.
  • Editor:
    • each stroke makes a new group and is one undo step;
    • Alt grows the group;
    • Escape cancels with nothing to undo;
    • X symmetry paints both sides;
    • hidden faces are skipped;
    • positions never change.
• Headless screenshot (lavapipe): three strokes on the starting sphere make three groups, each mirrored across X. The brush panel is trimmed. The screenshot came from a temporary local hack that runs the strokes at start; it is not in this PR.

Not done / limitations

• No "pick a group to paint with" eyedropper; Alt-grow covers extending one.
• Triangles are painted whole, by their centre, so the stroke edge follows the mesh, not a smooth curve.
• Not tested with a pen, on Windows or on macOS.

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Generated by Claude Code