engine: geometry.stitch, imported surfaces stitched for meshing (#507)

geometry.stitch {name, surface, tolerance} records a stitch; the
surfaces keep what was read (TaggedSurface::original) and are stitched
again from it whenever a surface is imported, the model opened, or a
stitch added or deleted (geometry.stitch.delete). Opening a model also
warns about merges that no longer apply.

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

crates/engine/src/cmd/mod.rs

@@ -31,6 +31,7 @@ pub mod replace;
 mod results;
 pub(crate) mod seeds;
 pub(crate) mod solve;
+pub(crate) mod stitch;
 mod symmetry;
 mod tags;
 pub mod update;
@@ -74,6 +75,8 @@ pub static ALL: &[CommandSpec] = &[
     virtual_faces::FACES_MERGE,
     virtual_faces::FACES_MERGE_DELETE,
     virtual_faces::FACE_ABSORB,
+    stitch::STITCH,
+    stitch::STITCH_DELETE,
     mesh::NODE_ADD,
     mesh::NODE_REMOVE,
     mesh::ELEMENT_ADD,