Re-land the robustness batch on main (#6): fillets running into an earlier round go to blend_corner (any radius) instead of the same-radius undo; the lip and drilled-hole regression tests move into tests/robustness.rs with the new command ids; measuring 5000 faces is checked for its counts, its time in an ignored perf test run alone

Part of #6

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/engine/src/lib.rs

@@ -9,9 +9,9 @@
 
 pub mod cmd;
 pub mod frames;
+pub mod fuzz;
 pub mod legacy_ids;
 pub mod licences;
-pub mod fuzz;
 pub mod params;
 pub mod recovery;
 pub mod sample;