Bound serial close so a dead port cannot hang the run

When a USB-serial adapter disappears mid-run, writes to the open handle
fail with PermissionError, but serial.close() can block forever. Cleanup
then wedged after the second port closed and Ctrl+C could not break out,
because the main thread was stuck inside a blocking Windows call.

Close on a daemon thread and give up after CLOSE_TIMEOUT_S, so an
abandoned port can never hold up the exit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

BE-RCU-ovp_test_csv.py

@@ -56,6 +56,7 @@ import minimalmodbus
 SLAVE_ID = 0x10          # all four devices share slave id 16
 BAUDRATE = 9600
 SERIAL_TIMEOUT = 2.0     # seconds
+CLOSE_TIMEOUT_S = 3.0    # give up on a port that will not close
 
 
 # ---------------------------------------------------------------------------
@@ -217,6 +218,17 @@ class Device:
         return reads
 
     def close(self):
+        # A USB adapter unplugged mid-run wedges serial.close() forever, and
+        # Ctrl+C cannot break a blocking Windows call. Close on a daemon
+        # thread so a dead port can never hold up the exit.
+        closer = threading.Thread(target=self._close_serial, daemon=True)
+        closer.start()
+        closer.join(CLOSE_TIMEOUT_S)
+        if closer.is_alive():
+            logger.warning("%s: %s did not close in %.0fs, abandoning it",
+                           self.name, self.port, CLOSE_TIMEOUT_S)
+
+    def _close_serial(self):
         try:
             self.inst.serial.close()
             logger.info("%s disconnected", self.name)