Restore max_new_tokens to 512 (4-bit gen is fast: ~25 tok/s on GPU)

generate.py

@@ -71,7 +71,7 @@ def _render(messages, tok) -> str:
 
 
 @GPU(duration=180)
-def generate(messages: list[dict], max_new_tokens: int = 256,
+def generate(messages: list[dict], max_new_tokens: int = 512,
              temperature: float = 0.2) -> str:
     """Generate an assistant reply for chat-format ``messages``."""
     if STUB: