STEP import takes up for Y-up files and names for the bodies' names (#1312)

file.insert_step and doc.open on a STEP file take `up` (turn a Y-up file so its up is +Z) and `names` (names for the imported bodies). The MCP tool and the app's control requests pass them through; an import without them works as before. Tests in the kernel's STEP reader, the engine, MCP and the file-load UI; docs/step-import.md and docs/mcp.md describe both.

docs/mcp.md

@@ -181,10 +181,10 @@ The `camera` of `screenshot` (the object `view.apply` and `view.save` take too)
 
 To look closely at one part of a large model, such as a STEP assembly, work in a headless session. These steps change only the open design in memory:
 
-1. Open the model with `solidcraft-cli mcp --in assembly.step` (or the `open` tool). `inspect_design` lists the bodies with their names and bounding boxes.
+1. Open the model with `solidcraft-cli mcp --in assembly.step` (or the `open` tool, which also takes `up` and `names`, see step 4 and [STEP import](step-import.md)). `inspect_design` lists the bodies with their names and bounding boxes.
 2. Remove what is in the way with `solid.remove`, e.g. `{"bodies": ["Cover"]}`.
 3. Cut the region out of the rest with `solid.box`, e.g. `{"corner": [40, 0, -45], "length": 20, "width": 50, "height": 45, "operation": "intersect", "targets": ["Housing", "Plate"]}`. Each listed body is trimmed to the box, and one that lies wholly outside the box is removed. The box itself is not kept.
-4. If the model lies on its back (many STEP files have +Y up), stand it Z up with `solid.move`, listing every body you keep, e.g. `{"bodies": ["Housing", "Plate"], "axis": [1, 0, 0], "angle": 90}`. It turns the bodies about the world origin, and a bare angle is in degrees. A turn of +90° about X takes +Y to +Z.
+4. If the model lies on its back (many STEP files have +Y up), open it with `up: "y"` (the `open` tool, `doc.open`, `file.insert_step`), which turns the file's bodies a quarter turn about X (Y becomes Z). One already open is stood Z up with `solid.move`, listing every body you keep, e.g. `{"bodies": ["Housing", "Plate"], "axis": [1, 0, 0], "angle": 90}`. It turns the bodies about the world origin, and a bare angle is in degrees. A turn of +90° about X takes +Y to +Z.
 5. Colour the parts that matter with `appearance.assign`, e.g. `{"bodies": ["Housing"], "color": "#3a7bd5"}`, and hide helper bodies or sketches with `browser.visibility`, e.g. `{"items": ["Plate"], "visible": false}`.
 6. Cut a section with `inspect.section`, e.g. `{"plane": {"origin": [50, 30, 0], "normal": [0, 1, 0]}}` or `{"plane": "XY", "at": 5}`.
 7. Take the picture with `screenshot`: a `camera` whose `target` is the centre of the region and whose eye is on the cut side, `source: "model"`, and `path`, `width` and `height` as needed. An orthographic picture is 0.8 × `distance` high, so a `distance` of about 1.5 times the region's size frames it. Look at the image, adjust `yaw`, `pitch` and `distance`, and shoot again. Headless, the picture is always a render of the model; bridged to the app, `source: "model"` asks for that render instead of the app's window.

docs/step-import.md

@@ -6,7 +6,18 @@ SolidCraft reads STEP (ISO 10303-21; AP203, AP214 and AP242 geometry) with its o
 - **Insert** into the current design: File → Insert STEP… or SOLID → INSERT (`file.insert_step {path}`).
 - **Drag and drop** a STEP file on the window: inserted when the design has features, opened otherwise.
 
-Read: manifold solid breps (with voids), faceted breps, shell-based surface models; planes, cylinders, cones, spheres, tori, B-spline and rational B-spline surfaces (with knots, Bézier, uniform, quasi-uniform), surfaces of revolution and linear extrusion, trimmed surfaces; lines, circles, ellipses, B-spline curves, polylines, trimmed/surface/seam curves; length and angle units (SI prefixes, conversion-based units such as inches); product names (body names come from the solid's name, else the product's); surface colours; assemblies (next-assembly-usage occurrences placed by item-defined transformations, and mapped items) — the product tree is kept in the feature for the components milestone. Files are untrusted: sizes, entity counts and nesting are capped and every kernel call is guarded. What cannot be read becomes a warning on the feature (a face left out gives an open body); not read yet: offset surfaces, pcurve-only edges.
+Read: manifold solid breps (with voids), faceted breps, shell-based surface models; planes, cylinders, cones, spheres, tori, B-spline and rational B-spline surfaces (with knots, Bézier, uniform, quasi-uniform), surfaces of revolution and linear extrusion, trimmed surfaces; lines, circles, ellipses, B-spline curves, polylines, trimmed/surface/seam curves; length and angle units (SI prefixes, conversion-based units such as inches); product names (body names come from the solid's name, else the product's; `names` below chooses); surface colours; assemblies (next-assembly-usage occurrences placed by item-defined transformations, and mapped items) — the product tree is kept in the feature for the components milestone. Files are untrusted: sizes, entity counts and nesting are capped and every kernel call is guarded. What cannot be read becomes a warning on the feature (a face left out gives an open body); not read yet: offset surfaces, pcurve-only edges.
+
+## Files from other CAD systems
+
+Files from different systems differ in which way is up and in what they call their solids. `doc.open`, `file.insert_step` and the MCP `open` tool take two options for STEP and IGES files; without them a file comes in as written.
+
+- **`up`**: `"z"` (the default) keeps the file's axes. Many systems write Y up, so such a part lies on its back in SolidCraft's Z-up space. `"up": "y"` turns the file's bodies a quarter turn about X (+90°: Y becomes Z) with one Move feature right after the Import, the one the window's "Stand it up" offer adds. The import and the turn are one undo step, the reply names the Move as `stand_up`, and only the bodies of the file turn, not the ones already in the design. The window offers to stand a part up after a STEP file opens unless the call said which way is up (`up: "z"` as well as `"y"`).
+- **`names`**: what the bodies are called. `"solid"` (the default) is the name of the solid in the file, and the product's name where the solid has none. Some systems fill a solid's name with the last feature that made it (`Cut-Extrude3`), so every body of an assembly has a name that says nothing about its part. `"product"` names each body after the product (the part) that holds it, and `"instance"` after the assembly occurrence (`Post-2`), or the product where the body is not in an occurrence. A name the file leaves out, or fills with a placeholder, falls back to the next one (instance, product, solid). Repeated names are numbered as any body names are (`Post`, `Post (2)`). The names are stored with the Import feature, so the design reopens with them. The default stays the solid's name because SolidCraft's own STEP export writes a body's name there.
+
+```json
+{"command": "file.insert_step", "params": {"path": "frame.step", "up": "y", "names": "instance"}}
+```
 
 ## Imported appearances