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/mcp/src/tools.rs

@@ -24,6 +24,11 @@ const MESHERS: &[(&str, &str, &str)] = &[
         "mesh.cells",
         "one mesh of a partitioned part, each cell with its own method {methods: {cell: method}, default, size, order}; cells joined across partitions (merged or tied)",
     ),
+    (
+        "hex_extrude",
+        "mesh.hex.extrude",
+        "hex layers from a section (faces, a neighbour's mesh or a 2D mesh) up to a target surface {source | source_set, to_surface, sides, direction | normal, layers | size, bias, body}",
+    ),
     ("mirror", "mesh.mirror", "reflect meshed bodies across a datum plane and join them on it {plane, bodies}"),
     ("block", "mesh.block", "structured hex block"),
     (