Surface GSMTAP BAD_FCS desync flag end-to-end

sigrok-iso7816-stream sets GSMTAP_FLAG_BAD_FCS (0x01) in the GSMTAP
header `res` byte on desynced TPDUs (mis-framed exchanges kept for
forensics). Propagate that flag through the whole pipeline so such
packets are explicitly surfaced instead of decoding as garbage:

- gsmtap.py: GSMTAP_FLAG_BAD_FCS const; GsmtapReceiver.read_packet()
  now returns (sub_type, data, flags) from the res byte.
- server/capture.py: GsmtapListener forwards the flags to the DB.
- pcap.py: _parse_gsmtap_hdr/_scan_gsmtap/extract_gsmtap and
  parse_pcap/parse_pcapng now yield a (ts, type, payload, flags)
  tuple, so the flag survives a PCAP/PCAPNG round-trip.
- server/server.py: PCAP import stores the flags.
- server/decode.py: gsmtap_flag_names() maps bit 0x01 to
  'bad FCS (desynced)'; merged into decoded errors for tpdu/atr/pps
  and rst/vcc, so the frontend ⚠ badge + Errors row flag the entry.

User-visible: desynced TPDUs show a ⚠ "bad FCS (desynced)" marker
in the trace and detail views. Tests: +4 (decode BAD_FCS markers,
PCAP flag round-trip); existing tests updated for the new tuple
arities. Version 1.22.0, SW cache simtrace-analyser-v32.

tests/test_decode.py

@@ -2340,6 +2340,22 @@ class TestLineEvents(unittest.TestCase):
         self.assertEqual(decode_sniff_msg(b'\x01\x00\x00', 'rst')['type'], 'rst')
         self.assertEqual(decode_sniff_msg(b'\x00\x01\x00', 'vcc')['type'], 'vcc')
 
+    def test_bad_fcs_flag_marks_tpdu(self):
+        from simtrace2_pysniff.server.decode import decode_sniff_msg
+        r = decode_sniff_msg(b'\x80\xf2\x00\x00\x00', 'tpdu', flags=0x01)
+        self.assertIn('bad FCS (desynced)', r['errors'])
+
+    def test_bad_fcs_flag_marks_rst(self):
+        from simtrace2_pysniff.server.decode import decode_sniff_msg
+        r = decode_sniff_msg(b'\x01\x00\x00', 'rst', flags=0x01)
+        self.assertIn('bad FCS (desynced)', r['errors'])
+        self.assertEqual(r['event'], 'reset asserted')
+
+    def test_no_fcs_flag_no_error(self):
+        from simtrace2_pysniff.server.decode import decode_sniff_msg
+        r = decode_sniff_msg(b'\x01\x00\x00', 'rst', flags=0)
+        self.assertNotIn('errors', r)
+
     def test_short_payload(self):
         r = self._dec('rst', '01')
         self.assertEqual(r['type'], 'rst')