engine: mesh.hex.octree command

Replaces the mesh with the octree all-hex mesh of a body, an imported surface or every body
of the open design, and tags boundary element faces with their B-rep faces so sets by
geometry resolve. Size, tolerance, refinement depth, element cap, attempts and optimiser
sweeps are validated and capped; the element cap defaults to the session limit
(`Limits::max_hex_elements`), which the hostile-params test keeps small. Body input and face
tagging are shared with mesh.hex.generate. The catalogue's Automatic Hex entry is ready.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/engine/src/cmd/mod.rs

@@ -9,6 +9,7 @@ mod geometry;
 mod hex;
 pub(crate) mod mesh;
 mod model;
+pub(crate) mod octree;
 mod results;
 pub(crate) mod solve;
 mod tags;
@@ -36,6 +37,7 @@ pub static ALL: &[CommandSpec] = &[
     mesh::SURFACE_CHECK,
     mesh::PLANAR,
     mesh::TET_GENERATE,
+    octree::HEX_OCTREE,
     block::BLOCK,
     hex::HEX_GENERATE,
     hex::HEX_SWEEP,