Engine: geometry.datum, geometry.datum.axis and geometry.partition

Datum planes by principal plane and offset, three points, or point and
normal; datum axes by principal axis, two points, or point and direction.
geometry.partition cuts the open design by a datum plane and keeps the
partition in the model, so geometry.open, file.open and undo cut the design
again. Requirements, forms and the Prepare entries are wired; features.md
lists them as ready.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/geometry.rs

@@ -64,7 +64,9 @@ fn open(s: &mut Session, p: &Value) -> Result<Value> {
     let path = super::file_param(s, p, "path", Access::Read)?;
     let bytes = super::file::read_file(&path, MAX_DESIGN_BYTES as u64)?;
     let source = SourceFile::of(super::file::absolute(&path), &bytes)?;
-    let design = open_design(&bytes)?;
+    let opened = open_design(&bytes)?;
+    // The model's partitions cut the design again (a new model has none).
+    let design = if new_model { opened } else { super::datum::partitioned(&opened, &s.model)? };
     let mut fit = fit(s, &design);
     if new_model {
         let mut model = Model::default();