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/schema.rs

@@ -217,6 +217,8 @@ pub enum Collection {
     /// A datum point.
     DatumPoint,
     Partition,
+    /// A face merge (`geometry.faces.merge`).
+    FaceMerge,
 }
 
 impl Collection {
@@ -242,6 +244,7 @@ impl Collection {
             Collection::DatumAxis => "datum axis",
             Collection::DatumPoint => "datum point",
             Collection::Partition => "partition",
+            Collection::FaceMerge => "face merge",
         }
     }
 }
@@ -271,6 +274,7 @@ impl Session {
             Collection::DatumAxis => Box::new(m.datums().iter().filter(|(_, d)| d.axis().is_some()).map(|(n, _)| n.as_str())),
             Collection::DatumPoint => Box::new(m.datums().iter().filter(|(_, d)| d.point().is_some()).map(|(n, _)| n.as_str())),
             Collection::Partition => Box::new(m.partitions().iter().map(|p| p.name.as_str())),
+            Collection::FaceMerge => Box::new(m.face_merges().iter().map(|f| f.name.as_str())),
         };
         names.take(MAX_NAMES).map(str::to_string).collect()
     }