Mesh: the swept cross-section's quad mesh as its own module

The schemes, interval constraints, curve nodes and region meshes of a
multi-level sweep's cross-section move out of layered::mesh into
hex::section, with the curve placement (seeds, projection onto walls)
passed in, so the cross-section can be meshed on its own.
region::mesh_structured gives a scheme's mesh or None where it would fold,
without falling back to the free mesher. No change in behaviour.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/mesh/src/hex/mod.rs

@@ -42,6 +42,7 @@ mod interval;
 mod layered;
 mod mapped;
 mod region;
+mod section;
 mod sweep;
 
 #[cfg(test)]