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/catalogue.rs

@@ -309,6 +309,14 @@ pub static CATALOGUE: &[Capability] = &[
         "Prepare",
         "Absorb tiny and sliver faces into a neighbour for meshing, one picked or every one Check Geometry found",
     ),
+    covering(
+        "geometry.face.split",
+        "Split Face",
+        Geometry,
+        "Prepare",
+        "Split a face along a datum plane for meshing only: two faces on its surface, the cut an edge between them",
+        &["geometry.face.split.delete"],
+    ),
     covering(
         "geometry.stitch",
         "Stitch",