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/blend.rs

@@ -298,6 +298,7 @@ fn blend(body: &Body, edges: &[Vec3], r: f64, shape: Shape, what: &str) -> Resul
 
 /// Constant-radius fillet of the edges nearest to the given points.
 pub fn fillet(body: &Body, edges: &[Vec3], radius: f64) -> Result<Body> {
+    body.require_brep("fillet")?;
     blend(body, edges, radius, Shape::Round, "fillet")
 }
 
@@ -305,6 +306,7 @@ pub fn fillet(body: &Body, edges: &[Vec3], radius: f64) -> Result<Body> {
 /// prism whose only face inside the body is the chamfer plane (so neighbouring chamfers meet in
 /// a mitre); the local operation is the fallback.
 pub fn chamfer(body: &Body, edges: &[Vec3], distance: f64) -> Result<Body> {
+    body.require_brep("chamfer")?;
     if !(distance.is_finite() && distance > 1e-6 && distance < 1e6) {
         return Err(KernelError::Invalid("chamfer size must be positive".into()));
     }