chore: remove dead extend stub, flatten single-file module dirs

- Delete src/modules/draw/modify/extend.rs (a 1-line placeholder with no
  tool()/logic; `extend::` was referenced nowhere) and its `mod extend;`.
- Flatten single-file module dirs to the modern flat style:
  src/command/mod.rs -> src/command.rs, src/snap/mod.rs -> src/snap.rs.
  Parent `mod command;`/`mod snap;` are unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/modules/draw/modify/extend.rs

@@ -1 +0,0 @@
-// Extend — dispatch handled by trim.rs dropdown. This file exists for module resolution.

src/modules/draw/modify/mod.rs

@@ -5,7 +5,6 @@ pub mod break_cmd;
 pub mod copy;
 pub mod delete;
 pub mod explode;
-mod extend;
 pub mod fillet;
 pub mod join;
 pub mod lengthen;