Colours in exports from materials and appearances; Appearance command

- Bodies export (STEP, 3MF) in the colour they are shown in: an appearance,
  else the physical material's look, else the colour the body was imported
  with (Document::body_color / painted). Physical materials used to export
  no colour at all.
- AppearanceCommand (Modify): a colour per body ("#rrggbb" or [r, g, b];
  null follows the material again), kept in the design and across renames.
- Material looks move to solvecraft-doc (material_color); the viewport draws
  appearance, material or imported colour, as exported.
- Engine test: brass and red ABS bodies export, re-import with their RGB, and
  keep it through open and export again.

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

crates/engine/src/cmd/browser.rs

@@ -219,6 +219,9 @@ fn rename_body(s: &mut Session, p: &Value) -> Result<Value> {
     if let Some(m) = d.materials.remove(&old) {
         d.materials.insert(name.to_string(), m);
     }
+    if let Some(c) = d.appearances.remove(&old) {
+        d.appearances.insert(name.to_string(), c);
+    }
     if let Some(c) = d.body_components.remove(&old) {
         d.body_components.insert(name.to_string(), c);
     }