3MF: export, and 3MF/STL import as mesh bodies

Export: ExportCommand/CLI --out/MCP export with .3mf write a 3MF Core package
([Content_Types].xml, _rels/.rels, 3D/3dmodel.model; millimetres), one
mesh object and build item per body, named after it, vertices shared and
welded watertight (T-junctions between faces split), base-material colours
when bodies carry one (STEP colours now ride on bodies). Tessellation as
for STL. File > Export 3MF.

Import: doc.open/drag-drop/CLI/MCP open accept .3mf and .stl; Insert Mesh
(ParaMeshInsertAlignCommand) adds them to a design as a MeshImport feature.
Kernel mesh bodies render, measure exactly, move and export; solid
features refuse them with a clear error. 3MF reading flattens components
and build transforms, converts units, reads colours; sizes, entries,
nesting and element counts are capped.

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

crates/kernel/src/step.rs

@@ -8,6 +8,9 @@ pub fn step_export(bodies: &[&Body], system: &str) -> Result<String> {
     if bodies.is_empty() {
         return Err(KernelError::Invalid("nothing to export".into()));
     }
+    for b in bodies {
+        b.require_brep("STEP export")?;
+    }
     guard("step export", || {
         let cs: Vec<_> = bodies.iter().map(|b| b.solid.compress()).collect();
         let models: out::StepModels<_, _, _> = cs.iter().collect();