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.