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

docs/mcp.md

@@ -59,7 +59,7 @@ Claude Desktop (`claude_desktop_config.json`), bridged to a running app:
 | `export` | `path`, `format?` (step, stl, stla, obj), `bodies?` | STEP or mesh export. |
 | `undo`, `redo` | — | Undo / redo one change. |
 | `new_design` | `name?` | Start an empty design. |
-| `open` | `path` | Open a `.solvecraft` design. |
+| `open` | `path` | Open a `.solvecraft` design, or a STEP file (`.step`/`.stp`) as a new design holding its bodies (an Import base feature). |
 | `save` | `path?` | Save the design (to its current file without `path`). |
 
 Resources: `solvecraft://design` (inspect JSON with measurements) and `solvecraft://commands`