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/mcp/src/tools.rs

@@ -190,8 +190,10 @@ pub fn tool_definitions() -> Vec<Value> {
         tool(
             "open",
             "Open a design",
-            "Open a .solvecraft design file (replaces the current design).",
-            obj(json!({"path": string("Path to a .solvecraft file")}), &["path"]),
+            "Open a .solvecraft design file, or a STEP file (.step/.stp: a new design whose bodies come from the file, as an \
+             Import base feature; insert into the current design with the FusionImportCommandFromToolbar command). Replaces \
+             the current design.",
+            obj(json!({"path": string("Path to a .solvecraft or .step/.stp file")}), &["path"]),
             false,
             true,
         ),