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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -2,6 +2,7 @@
 
 mod block;
 mod contact;
+pub(crate) mod datum;
 pub(crate) mod deck;
 pub(crate) mod define;
 mod edit;
@@ -32,6 +33,11 @@ pub static ALL: &[CommandSpec] = &[
     geometry::BODIES,
     geometry::TESSELLATE,
     geometry::IMPORT_MESH,
+    datum::DATUM_PLANE,
+    datum::DATUM_AXIS,
+    datum::DATUM_DELETE,
+    datum::PARTITION,
+    datum::PARTITION_DELETE,
     mesh::NODE_ADD,
     mesh::NODE_REMOVE,
     mesh::ELEMENT_ADD,