engine: mesh.hex.extrude, hex layers up to a target surface (#400)

The section comes from a set (a 2D mesh or element faces), the mesh already
on the source faces (tagged, or a neighbouring body's boundary lying on
them, whose nodes are shared so the meshes conform), or a fresh quad mesh
of one planar face. Target and side faces project through the tet mesher's
B-rep projection. Replaces the body's mesh like the other meshers, tags the
body's own source, target and side faces, and is on the MCP mesh tool as
hex_extrude.

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

crates/engine/src/cmd/mod.rs

@@ -8,6 +8,7 @@ pub(crate) mod deck;
 pub(crate) mod define;
 pub mod diagnostics;
 mod edit;
+mod extrude;
 pub(crate) mod file;
 pub(crate) mod forms;
 mod geometry;
@@ -84,6 +85,7 @@ pub static ALL: &[CommandSpec] = &[
     hex::HEX_MAPPED,
     hex::HEX_BLOCKS,
     project::HEX_PROJECT,
+    extrude::HEX_EXTRUDE,
     cells::CELLS,
     plan::PLAN,
     plan::APPLY,