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/index.ts

@@ -0,0 +1,32 @@
+export { useGame } from './useGame';
+export type { GameManager, GameOptions } from './types';
+
+export { useBoard } from './useBoard';
+export { useCursor } from './useCursor';
+export { useGameStatus } from './useGameStatus';
+export { useScore } from './useScore';
+
+export type { 
+  Coord, 
+  SudokuBoard, 
+  SudokuCell, 
+  Difficulty,
+  BoardManager 
+} from './useBoard/types';
+
+export type { 
+  CursorManager, 
+  CursorMode, 
+  GameKeyManager, 
+  KeyLogEntry 
+} from './useCursor/types';
+
+export type { 
+  GameStatus, 
+  GameStatusManager 
+} from './useGameStatus/types';
+
+export type { 
+  ScoreManager, 
+  TimerManager 
+} from './useScore/types';