bench(flatten): add FLATTEN metrics with extracted helpers

Moves the flatten projection fns and command helpers into modules/draw/modify/flatten.rs (behavior unchanged) and adds flatten_plan / flatten_scene_apply to the COMMANDS suite.

src/modules/draw/modify/mod.rs

@@ -6,6 +6,7 @@ pub mod break_cmd;
 pub mod copy;
 pub mod delete;
 mod entity_index;
+pub mod flatten;
 pub mod explode;
 pub mod fillet;
 pub mod geom;