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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/doc/src/eval.rs

@@ -569,7 +569,13 @@ fn blend_target(state: &ModelState, body: &Option<String>, edges: &[Vec3]) -> Re
             best = Some((i, d));
         }
     }
-    best.map(|(i, _)| i).ok_or_else(|| DocError::Invalid("there is no body to modify".into()))
+    best.map(|(i, _)| i).ok_or_else(|| {
+        if state.bodies.iter().any(|b| b.body.is_mesh()) {
+            DocError::Invalid("there is no solid body to modify (mesh bodies have no edges)".into())
+        } else {
+            DocError::Invalid("there is no body to modify".into())
+        }
+    })
 }
 
 fn revolve_axis(ss: &SolvedSketch, axis: &AxisRef) -> Result<(Vec2, Vec2)> {
@@ -1100,6 +1106,13 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             }
             Ok(())
         }
+        FeatureKind::MeshImport { meshes, .. } => {
+            for (name, b) in mesh_bodies(meshes)? {
+                let name = unique_body_name(st, &name);
+                st.bodies.push(ModelBody::new(name, b, f.id));
+            }
+            Ok(())
+        }
         FeatureKind::Import { step, .. } => {
             let imp = kernel::step_import_shared(step)?;
             for (k, b) in imp.bodies.iter().enumerate() {
@@ -1118,6 +1131,18 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
     }
 }
 
+/// Mesh bodies of a MeshImport feature.
+fn mesh_bodies(meshes: &[crate::MeshData]) -> Result<Vec<(String, Body)>> {
+    let mut out = Vec::with_capacity(meshes.len());
+    for m in meshes {
+        let positions: Vec<Vec3> = m.positions.as_chunks::<3>().0.iter().map(|c| Vec3::new(c[0], c[1], c[2])).collect();
+        let triangles: Vec<[u32; 3]> = m.triangles.as_chunks::<3>().0.to_vec();
+        let b = kernel::mesh_body(&positions, &triangles).map_err(|e| DocError::Invalid(format!("mesh `{}`: {e}", m.name)))?;
+        out.push((m.name.clone(), b.with_color(m.color)));
+    }
+    Ok(out)
+}
+
 /// `base`, or `base (2)`, `base (3)`… if a body already has that name.
 fn unique_body_name(st: &ModelState, base: &str) -> String {
     let base = if base.trim().is_empty() { "Body" } else { base.trim() };