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.

crates/engine/src/tests.rs

@@ -318,6 +318,7 @@ mod rim_cut;
 mod robustness;
 mod robustness_goldens;
 mod spiral_join;
+mod step_import_options;
 mod xz_plane;
 
 /// Plan area a round of radius `r` fills at the concave corner `x` where circles (c1, r1) and