engine: geometry.faces.merge, faces meshed as one (virtual topology, #505)

geometry.faces.merge {name, faces} records a face merge in the model;
it is applied after the partitions whenever the design is opened or cut
again, undoes through the journal and is saved with the model. Meshers,
mesh.plan (features and rings see merge:<name>; a merged face is
prismatic when its B-rep faces are), checks and the viewport see one
face, drawn in the warm colour. geometry.faces.merge.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

@@ -35,6 +35,7 @@ mod symmetry;
 mod tags;
 pub mod update;
 pub(crate) mod view;
+mod virtual_faces;
 pub mod xy;
 
 use crate::schema::ProblemKind;
@@ -70,6 +71,8 @@ pub static ALL: &[CommandSpec] = &[
     datum::DATUM_SUPPRESS,
     datum::PARTITION_SUPPRESS,
     datum::PARTITION_DELETE,
+    virtual_faces::FACES_MERGE,
+    virtual_faces::FACES_MERGE_DELETE,
     mesh::NODE_ADD,
     mesh::NODE_REMOVE,
     mesh::ELEMENT_ADD,