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/requirement.rs

@@ -32,6 +32,8 @@ pub enum Requirement {
     DatumPoint,
     /// A partition of the design.
     Partition,
+    /// A face merge.
+    FaceMerge,
     /// An edge seed.
     Seed,
     /// At least one mesh node.
@@ -79,6 +81,7 @@ impl Requirement {
             Requirement::DatumAxis => m.datums().values().any(|d| d.axis().is_some()),
             Requirement::DatumPoint => m.datums().values().any(|d| d.point().is_some()),
             Requirement::Partition => !m.partitions().is_empty(),
+            Requirement::FaceMerge => !m.face_merges().is_empty(),
             Requirement::Seed => !m.seeds().is_empty(),
             Requirement::Nodes => !m.nodes().is_empty(),
             Requirement::Mesh => has_elements,
@@ -119,6 +122,7 @@ impl Requirement {
             Requirement::DatumAxis => "a datum axis",
             Requirement::DatumPoint => "a datum point",
             Requirement::Partition => "a partition",
+            Requirement::FaceMerge => "a face merge",
             Requirement::Seed => "an edge seed",
             Requirement::Nodes => "mesh nodes",
             Requirement::Mesh | Requirement::Analysable => "a mesh",
@@ -149,6 +153,7 @@ impl Requirement {
             Requirement::DatumAxis => "geometry.datum.axis",
             Requirement::DatumPoint => "geometry.datum.point",
             Requirement::Partition => "geometry.partition",
+            Requirement::FaceMerge => "geometry.faces.merge",
             Requirement::Seed => "mesh.seed.edge",
             Requirement::Nodes => "mesh.node.add",
             Requirement::Mesh | Requirement::Analysable => "mesh.tet.generate",
@@ -206,6 +211,7 @@ impl crate::schema::Collection {
             C::DatumAxis => Requirement::DatumAxis,
             C::DatumPoint => Requirement::DatumPoint,
             C::Partition => Requirement::Partition,
+            C::FaceMerge => Requirement::FaceMerge,
         }
     }
 }
@@ -346,6 +352,7 @@ mod tests {
             C::DatumAxis,
             C::DatumPoint,
             C::Partition,
+            C::FaceMerge,
         ];
         let empty = Session::default();
         for c in all {