Ignore SIGHUP — survive terminal closure in background

__main__.py: add signal.SIGHUP → SIG_IGN before setting up
SIGINT/SIGTERM handlers. Sniffer no longer dies when the terminal
closes or the shell session ends.

README: add Background / Daemon section with nohup examples.

simtrace2_pysniff/__main__.py

@@ -131,6 +131,7 @@ def main():
             file_dumper.close()
         sys.exit(0)
 
+    signal.signal(signal.SIGHUP, signal.SIG_IGN)
     signal.signal(signal.SIGINT, on_signal)
     signal.signal(signal.SIGTERM, on_signal)