feat: prepare vim-sudoku for unified API implementation

- Archive previous implementation attempt (src -> src_failed)
- Create comprehensive IMPLEMENTATION_PLAN.md with phased approach
- Create IMPLEMENTATION.md to track implementation progress
- Update example app to match vim-maze patterns:
  - Simplify UI to desktop-only (no mobile support)
  - Add score metrics display (time, keystrokes, final score)
  - Update instructions for sudoku-specific keys (hjkl, i for edit)
  - Remove unnecessary mobile hooks

Phase 5 complete. Ready to begin Phase 10 (Setup & Core Types).

src_failed/useBoard/index.ts

@@ -0,0 +1,4 @@
+export { useBoard } from './useBoard';
+export type { BoardManager, SudokuBoard, SudokuCell, Coord, Difficulty } from './types';
+export { SudokuGenerator } from './SudokuGenerator';
+export { SudokuRenderer } from './SudokuRenderer';