Restore --skip-mhc arg, empty system prompt for testing

single_shot_inference.py

@@ -48,6 +48,7 @@ def parse_args():
     p = argparse.ArgumentParser(description='DSV4 Single-Shot Inference')
     p.add_argument('--no-inverse-rope', action='store_true', help='Skip inverse RoPE on attention output')
     p.add_argument('--skip-moe', action='store_true', help='Only use shared expert (skip routed)')
+    p.add_argument('--skip-mhc', action='store_true', help='Bypass mHC, use simple residual (diagnostic)')
     p.add_argument('--no-thinking', action='store_true', help='Force model to skip thinking (use <|EOT|> instead of thinking tokens)')
     p.add_argument('--max-tokens', type=int, default=512, help='Max new tokens to generate')
     p.add_argument('--prompt', type=str, default=None, help='Override prompt')