STEP export: AP242 file of our own, names, colours, components as assemblies

Our writer assembles the file: AP242 header and contexts, a product per part
(or per component, with NEXT_ASSEMBLY_USAGE_OCCURRENCE and item-defined
placements per occurrence), named solids, surface colours as styled items,
ADVANCED_FACE for advanced breps. truck now writes only each body's B-rep,
which we copy in; identical geometric entities are written once and shared.
Designs with components export as assemblies (bodies in component frames).

step_validate also rejects unreferenced non-root entities. Tests: the sample
plate and a component assembly export, validate and read back with no
warnings and the same volume, area and face counts; the oracle replay checks
area and faces too (39 rebuildable cases pass). An independent Part 21
parser (step-p21, run outside the repo) accepts our exports.

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

crates/kernel/src/step_in/tests.rs

@@ -309,10 +309,13 @@ fn exponential_assemblies_are_bounded() {
 
 #[test]
 fn validator_catches_broken_files() {
-    assert!(step_validate(&format!("{HEAD}#1=A(#2);\n#2=B();\n{TAIL}")).is_ok());
-    assert!(step_validate(&format!("{HEAD}#1=A(#2);\n#2=B();\n#2=C();\n{TAIL}")).unwrap_err().contains("duplicate"));
+    assert!(step_validate(&format!("{HEAD}#1=SHAPE_DEFINITION_REPRESENTATION(#2);\n#2=B();\n{TAIL}")).is_ok());
+    assert!(
+        step_validate(&format!("{HEAD}#1=SHAPE_DEFINITION_REPRESENTATION(#2);\n#2=B();\n#3=C();\n{TAIL}")).unwrap_err().contains("not referenced")
+    );
+    assert!(step_validate(&format!("{HEAD}#1=SHAPE_DEFINITION_REPRESENTATION(#2);\n#2=B();\n#2=C();\n{TAIL}")).unwrap_err().contains("duplicate"));
     assert!(step_validate(&format!("{HEAD}#1=A((#7));\n{TAIL}")).unwrap_err().contains("undefined"));
-    assert!(step_validate(&format!("{HEAD}#1=A();\n#5=B();\n{TAIL}")).unwrap_err().contains("dense"));
+    assert!(step_validate(&format!("{HEAD}#1=SHAPE_DEFINITION_REPRESENTATION(#5);\n#5=B();\n{TAIL}")).unwrap_err().contains("dense"));
 }
 
 #[test]