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/useScore/useTimer.ts

@@ -0,0 +1,45 @@
+import { useState, useEffect, useRef } from 'react';
+import type { TimerManager } from './types';
+
+export function useTimer(): TimerManager {
+  const [timeValue, setTimeValue] = useState<number>(0);
+  const intervalRef = useRef<NodeJS.Timeout | null>(null);
+  const startTimeRef = useRef<number>(0);
+
+  const startTimer = () => {
+    if (intervalRef.current) return;
+    
+    startTimeRef.current = Date.now() - timeValue;
+    intervalRef.current = setInterval(() => {
+      setTimeValue(Date.now() - startTimeRef.current);
+    }, 100);
+  };
+
+  const stopTimer = () => {
+    if (intervalRef.current) {
+      clearInterval(intervalRef.current);
+      intervalRef.current = null;
+    }
+  };
+
+  const resetTimer = () => {
+    stopTimer();
+    setTimeValue(0);
+    startTimeRef.current = 0;
+  };
+
+  useEffect(() => {
+    return () => {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current);
+      }
+    };
+  }, []);
+
+  return {
+    timeValue,
+    startTimer,
+    stopTimer,
+    resetTimer
+  };
+}