STEP: open/insert/drag-drop

Import base feature (FeatureKind::Import, timeline type BaseFeature) holding
the STEP text and the file's product tree; bodies named from the file.
doc.open accepts .step/.stp (any case) as a new design; Insert STEP
(FusionImportCommandFromToolbar) adds one to the current design; File menu,
SOLID > INSERT, drag-and-drop (insert into a non-empty design, else open),
'solvecraft part.step', MCP open. Features (fillet, hole, sketch-on-face
cut, shell) work on imported bodies. Imports are cached so a file is read
once per command. Measure tightens its chord tolerance for small radii.

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

crates/ui-egui/src/toolbar.rs

@@ -127,6 +127,10 @@ fn file_menu(app: &mut SolveApp, ctx: &egui::Context) {
                 }
                 close = true;
             }
+            if item(ui, "Insert STEP…", "") {
+                app.start("FusionImportCommandFromToolbar");
+                close = true;
+            }
             if item(ui, "Save", "Ctrl+S") {
                 save(app);
                 close = true;