engine: geometry.face.split, a face split along a datum plane (#508)

geometry.face.split {name, face, datum} records a face split; it is
applied after the merges whenever the design is opened or cut again,
follows its datum (which it holds), undoes, and is saved with the
model. geometry.face.split.delete removes it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -76,6 +76,8 @@ pub static ALL: &[CommandSpec] = &[
     virtual_faces::FACES_MERGE,
     virtual_faces::FACES_MERGE_DELETE,
     virtual_faces::FACE_ABSORB,
+    virtual_faces::FACE_SPLIT,
+    virtual_faces::FACE_SPLIT_DELETE,
     stitch::STITCH,
     stitch::STITCH_DELETE,
     mesh::NODE_ADD,