Engine: mesh.seed.edge, mesh.seed.clear and mesh.seed.list

Seed design edges by their persistent names with a count or a size, a bias
and a constraint; one undo step each, saved with the model. Seeds on edges a
reopened design lacks are reported as stale.

Part of #230.

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

crates/engine/src/cmd/mod.rs

@@ -18,6 +18,7 @@ mod mirror;
 mod model;
 pub(crate) mod octree;
 mod results;
+pub(crate) mod seeds;
 pub(crate) mod solve;
 mod symmetry;
 mod tags;
@@ -57,6 +58,9 @@ pub static ALL: &[CommandSpec] = &[
     hex::HEX_MAPPED,
     hex::HEX_BLOCKS,
     cells::CELLS,
+    seeds::SEED_EDGE,
+    seeds::SEED_CLEAR,
+    seeds::SEED_LIST,
     mirror::MIRROR,
     tags::TAG,
     tags::TAG_CLEAR,