mesh: hex extrusion of a quad section up to a target surface (#400)

Each source node follows its own path (a direction or the section normal)
to the target, found by Newton on the target's tangent plane; layers are
graded by a bias; side faces, when given, hold the boundary nodes of every
layer (alternating projections at the target), and interior nodes take the
boundary-residual correction. Source nodes are kept, so a section taken from
a neighbouring mesh conforms with it.

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

crates/mesh/src/hex/mod.rs

@@ -40,6 +40,7 @@ mod arrangement;
 mod brep;
 mod cells;
 mod decompose;
+mod extrude_to;
 pub(crate) mod geom;
 mod interval;
 mod layered;
@@ -54,6 +55,7 @@ mod tests;
 use std::collections::HashMap;
 
 pub use cells::{Cell, CellsMesh, GROWTH, MAX_CELLS, hex_mesh_cells, hex_mesh_swept_cells};
+pub use extrude_to::{ExtrudeBoundary, ExtrudeLayers, ExtrudePath, ExtrudeRole, ExtrudeSpec, Extruded, MAX_BIAS, MAX_LAYERS, extrude_to_surface};
 pub use geom::Surface;
 
 use crate::seeds::EdgeSeed;