Stub out previous undo implementation; update docs.

- Remove outdated `undo-state.md`
- Add two code quality/optimization reports that were used to guide previous work:
  - `code-report.md` (optimization)
  - `code-report-quality.md` (stability and code health)
- Add `themes.md`.
- Update undo system docs and roadmap.

UndoSystem.h

@@ -50,6 +50,4 @@ private:
 
 	Buffer *buf_;
 	UndoTree &tree_;
-	// Internal hint for Delete batching: whether next Append() should prepend
-	bool pending_prepend_ = false;
 };