engine: geometry.stitch, imported surfaces stitched for meshing (#507)

geometry.stitch {name, surface, tolerance} records a stitch; the
surfaces keep what was read (TaggedSurface::original) and are stitched
again from it whenever a surface is imported, the model opened, or a
stitch added or deleted (geometry.stitch.delete). Opening a model also
warns about merges that no longer apply.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/schema.rs

@@ -219,6 +219,8 @@ pub enum Collection {
     Partition,
     /// A face merge (`geometry.faces.merge`).
     FaceMerge,
+    /// A stitch (`geometry.stitch`).
+    Stitch,
 }
 
 impl Collection {
@@ -245,6 +247,7 @@ impl Collection {
             Collection::DatumPoint => "datum point",
             Collection::Partition => "partition",
             Collection::FaceMerge => "face merge",
+            Collection::Stitch => "stitch",
         }
     }
 }
@@ -275,6 +278,7 @@ impl Session {
             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())),
+            Collection::Stitch => Box::new(m.stitches().iter().map(|f| f.name.as_str())),
         };
         names.take(MAX_NAMES).map(str::to_string).collect()
     }