Fix score formula to match design specification

The score computation used wrong weights and terms compared to DESIGN.md.
Updated to: enemies_killed*100 + levels_completed*500 - deaths*200 - time_elapsed

Closes #6

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/game/stats.c

@@ -8,10 +8,10 @@ void stats_reset(GameStats *s) {
 }
 
 void stats_update_score(GameStats *s) {
-    int score = s->levels_completed * 100
-              + s->enemies_killed * 10
-              - s->deaths * 25
-              + s->pickups_collected * 5;
+    int score = s->enemies_killed * 100
+              + s->levels_completed * 500
+              - s->deaths * 200
+              - (int)s->time_elapsed;
     s->score = score > 0 ? score : 0;
 }