Mesh: the triangle primitive for three-cornered sweep regions

A cross-section region with three corners (a fillet's quarter disk, a
pocket corner cut off by a partition) was left to the free all-quad
mesher. It now gets midpoint subdivision's triangle primitive (Tam and
Armstrong 1991; Li, Armstrong, McKeag 1997): lines from a centre to a
node on each side split it into three mapped blocks, the interior then
smoothed and every quadrilateral checked (a fold falls back to free).
Interval assignment gains dominance constraints (a side shorter than
the other two together) so the split always exists.

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

crates/mesh/src/quad/mod.rs

@@ -71,7 +71,7 @@ mod cdt;
 mod domain;
 mod free;
 mod guard;
-mod smooth;
+pub(crate) mod smooth;
 pub mod tfi;
 
 pub use domain::{MAX_BOUNDARY_POINTS, MAX_LOOPS};