Meshers replace the mesh of the bodies they mesh, and only theirs

mesh.tet.generate and mesh.planar stacked a second mesh on a meshed body;
mesh.hex.*, mesh.hex.octree and mesh.cells wiped every body's mesh. All of
them, and mesh.mirror for its image bodies, now go through replace::clear:
the body's old elements and the nodes only they use go, other bodies stay.
The reply reports replaced {body, elements, nodes}, stale_sets (sets that
listed the old labels) and results_cleared; the command is one undo step.
replace::MESHERS names the bodies each mesher fills, and replacement()
says what a run would replace, for the app to ask first. mesh.block stays
additive: a body is built from several blocks. Refs #267.

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

crates/engine/src/cmd/mod.rs

@@ -17,6 +17,7 @@ pub use mesh::analytic_surfaces;
 mod mirror;
 mod model;
 pub(crate) mod octree;
+pub mod replace;
 mod results;
 pub(crate) mod seeds;
 pub(crate) mod solve;