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

README.md

@@ -29,7 +29,10 @@ storytold "craft" family.
   changed feature; it can be rolled back, and features can be suppressed, renamed, edited and
   deleted.
 - **Parameters** with unit-aware expressions (`2 * width + 5 mm`, `angle / 2`).
-- **Files**: designs as JSON (`.solvecraft`), export to STEP, STL (binary/ASCII) and OBJ.
+- **Files**: designs as JSON (`.solvecraft`), export to STEP, STL (binary/ASCII) and OBJ;
+  STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) as a base feature
+  that later features build on — open a `.step`/`.stp` file, insert one into a design, or drop it
+  on the window ([docs/step-import.md](docs/step-import.md)).
 - **Desktop app** (egui + wgpu): toolbar with workspace tabs, browser, timeline, 3D viewport with
   view cube and navigation bar, sketch tools, feature dialogs, command palette, and a JSON-lines
   control channel so agents and tests can drive it.
@@ -45,6 +48,8 @@ cargo run --release -p solvecraft -- --sample          # desktop app with a samp
 cargo run --release -p solvecraft-cli -- commands      # the command registry
 cargo run --release -p solvecraft-cli -- run examples/bracket.json --out bracket.step
 cargo run --release -p solvecraft-cli -- snapshot examples/bracket.json --out bracket.png
+cargo run --release -p solvecraft -- part.step          # open a STEP file as a new design
+cargo run --release -p solvecraft-cli -- eval part.step # measure an imported STEP file
 cargo xtask ci                                          # fmt, clippy, tests, assets, layers, wasm
 ```
 
@@ -83,7 +88,7 @@ configuration and an example session: [docs/mcp.md](docs/mcp.md).
 | L0 | `geom` | vectors, planes, profiles, meshes and their measures |
 | L1 | `sketch`, `kernel`, `render` | constraint solver and profiles; the B-rep kernel boundary (truck); view math and CPU rasterizer |
 | L2 | `doc` | parameters, expressions, the feature timeline and its incremental evaluation |
-| L3 | `io` | design files, STL/OBJ/STEP export |
+| L3 | `io` | design files, STL/OBJ/STEP export, STEP import features |
 | L4 | `engine` | the session and the command registry (everything is a command) |
 | L5 | `ui-egui`, `mcp` | the swappable desktop front end; the MCP server (headless or bridged to the app) |
 | apps | `solvecraft`, `solvecraft-cli` | desktop app, headless CLI |