Face colours and opacity in STEP and 3MF export (and STEP import)

- Bodies carry a Paint for export (opacity, per-face colours), set by
  Document::painted from the appearance model. The paint comes from a
  face appearance, a body/component appearance's opacity or a material
  look (Glass), and falls back to the paint the body was imported with.
- STEP: faces with colours of their own get a STYLED_ITEM on their
  ADVANCED_FACE. See-through looks add SURFACE_STYLE_TRANSPARENT.
  Differently coloured pieces are not merged into one face.
- STEP import reads face styles and transparency. The reader tracks the
  file face of each kernel face (bands split in two keep it), so imported
  face colours land on the right faces; the design shows them through
  face_colors.
- 3MF: per-triangle colours (pid/p1 into the base materials, carried
  through welding) and the object's alpha (#RRGGBBAA).
- Tests: a blue half-transparent cube with a red top exports, re-imports
  with its opacity and the red top face, and keeps it when opened and
  exported again. 3MF writes the alpha and the face entry.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/lib.rs

@@ -36,7 +36,7 @@ mod sweep_path;
 mod topo;
 
 pub use blend::{chamfer, fillet};
-pub use body::{Body, EdgeInfo, FaceInfo};
+pub use body::{Body, EdgeInfo, FaceInfo, FacePaint, Paint};
 pub use build::{PathSeg, box_solid, cylinder, extrude, extrude_tapered, loft, revolve, sphere, sweep, torus};
 pub use helix::{loft_to_point, sweep_helix};
 pub use measure::{BodyMeasure, measure};