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/ui-egui/src/toolbar.rs

@@ -131,6 +131,10 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                 app.start("FusionImportCommandFromToolbar");
                 close = true;
             }
+            if item(ui, "Insert Mesh…", "") {
+                app.start("ParaMeshInsertAlignCommand");
+                close = true;
+            }
             if item(ui, "Save", "Ctrl+S") {
                 save(app);
                 close = true;
@@ -148,6 +152,10 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                 export(app, "step");
                 close = true;
             }
+            if item(ui, "Export 3MF…", "") {
+                export(app, "3mf");
+                close = true;
+            }
             if item(ui, "Export OBJ…", "") {
                 export(app, "obj");
                 close = true;