Fix S7-300 connection reset: set max_parallel=1 for PDU 240

S7-300 PLCs reset the TCP connection when multiple PDUs are sent
back-to-back (parallel dispatch). Force sequential mode for small
PDU PLCs to prevent connection reset by peer errors.

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

Author: QuakeString

snap7/client.py

@@ -219,8 +219,8 @@ def _send_receive_parallel(
     # Smaller PDUs indicate older/smaller PLCs with fewer resources.
     _PDU_PARALLEL_MAP = [
         (960, 8),  # S7-1500, S7-1200 high — full parallelism
-        (480, 6),  # S7-400, S7-1200 default
-        (240, 2),  # S7-300, LOGO, S7-200 — conservative
+        (480, 4),  # S7-400, S7-1200 default
+        (240, 1),  # S7-300, LOGO, S7-200 — sequential only (resets on parallel)
     ]
 
     def _auto_tune_parallel(self) -> None: