fix: reduce step penalty from -1 to -0.1

Step penalty of -1 was punishing the model for surviving longer,
making death the optimal strategy. Reduced to -0.1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tetris_env/server/game_engine.py

@@ -37,7 +37,7 @@ LINE_REWARDS = {
     4: 1500,  # "Tetris!" — the dream
 }
 
-STEP_PENALTY = -1
+STEP_PENALTY = -0.1
 HOLE_PENALTY_MULT = -5
 GAME_OVER_PENALTY = -500