feat: implement Phase 10 - minimal skeleton with empty grid

Create clean src/ structure with minimal viable implementation:

Core Files:
- src/types.ts: Basic type definitions (Difficulty, GameStatus, GameManager)
- src/index.ts: Main package exports
- src/game.css: Copied from src_old for styling
- src/useGame.ts: Orchestrator with mocked GameManager API
- src/useBoard/useBoard.ts: Renders empty 9x9 sudoku grid

Implementation Details:
- useBoard renders 81 cells with sudoku-grid/sudoku-cell classes
- Each cell has data-r and data-c attributes for identification
- useGame provides full GameManager interface to satisfy example app
- Real implementations: gameStatus, keyLog tracking, game controls
- Mocked implementations: cursor ({}), scoreManager (zeros/null)
- Supports optional platformHook parameter

Build Status:
- TypeScript compiles cleanly with no errors
- Example app builds successfully
- Ready for browser testing with empty grid display

Phase 10 complete. Next: Phase 20 (full useBoard with puzzle generation).

src/useBoard/useBoard.ts

@@ -0,0 +1,37 @@
+import { useRef } from 'react';
+
+export function useBoard() {
+  const containerRef = useRef<HTMLDivElement | null>(null);
+
+  function renderBoard() {
+    const container = containerRef.current;
+    if (!container) return;
+
+    container.innerHTML = '';
+    
+    const sudokuDiv = document.createElement('div');
+    sudokuDiv.id = 'sudoku';
+
+    const grid = document.createElement('div');
+    grid.className = 'sudoku-grid';
+
+    for (let i = 0; i < 81; i++) {
+      const r = Math.floor(i / 9);
+      const c = i % 9;
+      const cell = document.createElement('div');
+      cell.className = 'sudoku-cell';
+      cell.dataset.r = String(r);
+      cell.dataset.c = String(c);
+      cell.textContent = '';
+      grid.appendChild(cell);
+    }
+
+    sudokuDiv.appendChild(grid);
+    container.appendChild(sudokuDiv);
+  }
+
+  return {
+    containerRef,
+    renderBoard,
+  };
+}