Engine: mesh.mirror reflects meshed bodies across a plane

The bodies' elements are reflected across a datum plane (or a point and
normal) with fieldcraft_mesh::mirror and joined on it. The image is tagged as
the design's body at the reflected place (found by volume and centroid, or
named in `as`), its faces with that body's faces under them, so sets by face
name reach both halves. Half a bar meshed hex8 or hex20 and mirrored has the
whole bar's node and element counts and volume, and its bending deflection to
1e-12; tet4 and tet10 mirror valid and conforming.

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

crates/engine/src/cmd/mod.rs

@@ -14,6 +14,7 @@ mod hex;
 pub(crate) mod mesh;
 
 pub use mesh::analytic_surfaces;
+mod mirror;
 mod model;
 pub(crate) mod octree;
 mod results;
@@ -55,6 +56,7 @@ pub static ALL: &[CommandSpec] = &[
     hex::HEX_MAPPED,
     hex::HEX_BLOCKS,
     cells::CELLS,
+    mirror::MIRROR,
     tags::TAG,
     tags::TAG_CLEAR,
     model::UNITS,