feat: add intermediate efficiency calculation for real-time feedback

- While keystrokes ≤ optimalSteps: show progress (keystrokes/optimal * 100)
  - Players see efficiency climbing from 0→100% as they play well
  - Provides real-time feedback during navigation
- After keystrokes > optimalSteps: show degradation (optimal/keystrokes * 100)
  - Efficiency smoothly decreases below 100%
- Examples with 100 optimal steps:
  - At 25 keystrokes: 25% (progress)
  - At 50 keystrokes: 50% (halfway)
  - At 100 keystrokes: 100% (optimal reached!)
  - At 120 keystrokes: 83% (slightly over)
  - At 200 keystrokes: 50% (double the optimal)

src/useScore/useScore.ts

@@ -51,9 +51,15 @@ export function useScore({ board, hero, gameStatusManager, keyManager, gameOver
 
   const optimalSteps = optimalRef.current;
   // Efficiency: 100% = optimal play, 0% = worst case
-  // Formula: (optimalSteps / keystrokes) * 100, clamped to 0-100
-  const efficiency = optimalSteps > 0 && keystrokes > 0 
-    ? Math.max(0, Math.min(100, Math.round((optimalSteps / keystrokes) * 100)))
+  // - If keystrokes <= optimalSteps: intermediate calc showing progress toward goal
+  //   Formula: (keystrokes / optimalSteps) * 100 (shows how far along optimal path)
+  // - If keystrokes > optimalSteps: inverse calc showing how much over optimal
+  //   Formula: (optimalSteps / keystrokes) * 100 (shows excess usage)
+  // Result clamped to 0-100
+  const efficiency = optimalSteps > 0 && keystrokes > 0
+    ? keystrokes <= optimalSteps
+      ? Math.round((keystrokes / optimalSteps) * 100)
+      : Math.max(0, Math.round((optimalSteps / keystrokes) * 100))
     : 100;
 
   // Check custom gameOver condition if provided