feat: add configurable removedCells option and comprehensive documentation

- Add removedCells to GameOptions for custom puzzle sizes
- removedCells overrides difficulty default when provided
- Maintain backward compatibility with all defaults
- Complete IMPLEMENTATION.md with Iteration 16 and summary
- Completely rewrite README.md with comprehensive documentation:
  - Complete API reference (GameOptions, GameManager, CursorManager, ScoreManager)
  - Game states and state machine flow
  - Full VIM controls reference (movement, editing, hints, game control)
  - Cell state rules and visual feedback
  - Detailed scoring formula with examples
  - Game-over conditions (time and hint limits)
  - Hint system priority and penalties
  - Configuration examples for different play styles
  - Platform hook integration guide
- Mark implementation as COMPLETE and PRODUCTION READY

src/useBoard/SudokuGenerator.ts

@@ -14,14 +14,14 @@ export class SudokuGenerator {
     this.generate('medium');
   }
 
-  generate(difficulty: Difficulty = 'medium'): void {
+  generate(difficulty: Difficulty = 'medium', customRemovedCells?: number): void {
     this.solution = this.generateSolvedBoard();
     
     if (!this.validateFullBoard(this.solution)) {
       throw new Error('Generated invalid solved board');
     }
     
-    this.puzzle = this.makePuzzle(this.solution, difficulty);
+    this.puzzle = this.makePuzzle(this.solution, difficulty, customRemovedCells);
   }
 
   getSolution(): string {
@@ -74,12 +74,12 @@ export class SudokuGenerator {
     return board.flat().join('');
   }
 
-  private makePuzzle(solution: string, difficulty: Difficulty): string {
+  private makePuzzle(solution: string, difficulty: Difficulty, customRemovedCells?: number): string {
     if (solution.length !== 81 || !/^[1-9]+$/.test(solution)) {
       throw new Error('Invalid solved board: must be 81 digits 1–9');
     }
 
-    const removeCount = CELLS_TO_REMOVE[difficulty];
+    const removeCount = customRemovedCells ?? CELLS_TO_REMOVE[difficulty];
     const board = solution.split('');
 
     const indices = Array.from({ length: 81 }, (_, i) => i);