Cover imported STEP colours through edits and round trips

docs/step-import.md

@@ -8,6 +8,19 @@ SolidCraft reads STEP (ISO 10303-21; AP203, AP214 and AP242 geometry) with its o
 
 Read: manifold solid breps (with voids), faceted breps, shell-based surface models; planes, cylinders, cones, spheres, tori, B-spline and rational B-spline surfaces (with knots, Bézier, uniform, quasi-uniform), surfaces of revolution and linear extrusion, trimmed surfaces; lines, circles, ellipses, B-spline curves, polylines, trimmed/surface/seam curves; length and angle units (SI prefixes, conversion-based units such as inches); product names (body names come from the solid's name, else the product's); surface colours; assemblies (next-assembly-usage occurrences placed by item-defined transformations, and mapped items) — the product tree is kept in the feature for the components milestone. Files are untrusted: sizes, entity counts and nesting are capped and every kernel call is guarded. What cannot be read becomes a warning on the feature (a face left out gives an open body); not read yet: offset surfaces, pcurve-only edges.
 
+## Imported appearances
+
+Imported body colours and face colours are retained through later features. Body
+colours remain the fallback under explicit design appearances and materials; face
+colours follow persistent face names. A removed face has no colour assignment to
+display on the resulting body.
+
+The RGB import regression in the engine checks a STEP file with separate red,
+green and blue bodies: fillet and move the red body, cut the blue body, join an
+extension to the green body, undo/redo,
+save and reopen the design, then export and reopen STEP. It checks all three body
+colours and the red body's separately coloured top face at each stage.
+
 ## IGES
 
 IGES 5.3 files (`.igs`/`.iges`) open, insert and drop the same way: `crates/kernel/src/iges_in.rs` (written from the published specification) restates the file as STEP and the STEP reader reads it, so the feature holds STEP text. Read: manifold solid B-reps (186) with their shells, faces, loops, edge and vertex lists; trimmed (144) and bounded (143) surfaces, and bare B-spline patches, sewn into a solid by their shared edges (an open body when they do not close); lines, circular arcs, ellipses, composite curves, rational B-spline curves; planes (108, 190), cylinders, cones, spheres, tori, rational B-spline surfaces, surfaces of revolution and tabulated cylinders; transformation matrices; units; colours (314 and the predefined numbers); body names (name properties, else labels). Not read yet: ruled and offset surfaces, curves on surfaces given only in parameter space. Export (`file.export` to `.igs`) writes each body as a manifold solid B-rep on the same surfaces a STEP export uses, with its name and colour.