engine: mesh.size.* commands, honoured by every mesher (#435)

mesh.size.set adds a size control on faces, bodies, a box or a sphere;
mesh.size.growth sets the growth rate; mesh.size.clear and
mesh.size.list. The tet mesher bakes the controls into its graded field,
the hex meshers, cells and the octree take the graded RegionSize, and
the block projection and mesh plans divide their edges by it.

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

crates/engine/src/schema.rs

@@ -223,6 +223,8 @@ pub enum Collection {
     Stitch,
     /// A face split (`geometry.face.split`).
     FaceSplit,
+    /// A size control (`mesh.size.set`).
+    SizeControl,
 }
 
 impl Collection {
@@ -251,6 +253,7 @@ impl Collection {
             Collection::FaceMerge => "face merge",
             Collection::Stitch => "stitch",
             Collection::FaceSplit => "face split",
+            Collection::SizeControl => "size control",
         }
     }
 }
@@ -283,6 +286,7 @@ impl Session {
             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())),
             Collection::FaceSplit => Box::new(m.face_splits().iter().map(|f| f.name.as_str())),
+            Collection::SizeControl => Box::new(m.size_controls().keys().map(String::as_str)),
         };
         names.take(MAX_NAMES).map(str::to_string).collect()
     }