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')

tests/test_gsmtap.py

@@ -9,7 +9,7 @@ class TestGsmtapReceiver(unittest.TestCase):
     def test_read_after_close_returns_none(self):
         r = GsmtapReceiver(bind_port=0)
         r.close()
-        self.assertEqual(r.read_packet(), (None, None))
+        self.assertEqual(r.read_packet(), (None, None, 0))
 
 
 if __name__ == '__main__':

tests/test_pcap_import.py

@@ -50,7 +50,7 @@ class TestParsePcap(unittest.TestCase):
             (_gsmtap_hdr(GSMTAP_SIM_APDU), b'\x80\xf2\x00\x00\x00', 2.5),
         ]
         data = build_pcap(packets)
-        parsed = [(round(ts, 1), t, payload) for ts, t, payload in parse_pcap(data)]
+        parsed = [(round(ts, 1), t, payload) for ts, t, payload, _f in parse_pcap(data)]
         self.assertEqual(parsed, [
             (1.0, 'atr', b'\x3b\x00'),
             (2.5, 'tpdu', b'\x80\xf2\x00\x00\x00'),
@@ -63,7 +63,7 @@ class TestParsePcap(unittest.TestCase):
             (_gsmtap_hdr(GSMTAP_SIM_VCC_EVENT), b'\x00\x00\x00', 1.5),
         ]
         data = build_pcap(packets)
-        parsed = [(round(ts, 1), t, payload) for ts, t, payload in parse_pcap(data)]
+        parsed = [(round(ts, 1), t, payload) for ts, t, payload, _f in parse_pcap(data)]
         self.assertEqual(parsed, [
             (1.0, 'rst', b'\x01\x00\x00'),
             (1.5, 'vcc', b'\x00\x00\x00'),
@@ -77,6 +77,18 @@ class TestParsePcap(unittest.TestCase):
         data = build_pcap(packets)
         self.assertEqual(list(parse_pcap(data)), [])
 
+    def test_bad_fcs_flag_roundtrip(self):
+        # GSMTAP_FLAG_BAD_FCS (res byte 0x01) survives a PCAP round-trip.
+        from simtrace2_pysniff.gsmtap import GSMTAP_FLAG_BAD_FCS
+        hdr = bytearray(_gsmtap_hdr(GSMTAP_SIM_APDU))
+        hdr[15] |= GSMTAP_FLAG_BAD_FCS
+        packets = [(bytes(hdr), b'\x80\xf2\x00\x00', 1.0),
+                   (_gsmtap_hdr(GSMTAP_SIM_APDU), b'\x80\xf2\x00\x00', 2.0)]
+        data = build_pcap(packets)
+        parsed = list(parse_pcap(data))
+        self.assertEqual(parsed[0], (1.0, 'tpdu', b'\x80\xf2\x00\x00', GSMTAP_FLAG_BAD_FCS))
+        self.assertEqual(parsed[1], (2.0, 'tpdu', b'\x80\xf2\x00\x00', 0))
+
 
 class TestParsePcapng(unittest.TestCase):
     def test_parse(self):
@@ -84,7 +96,7 @@ class TestParsePcapng(unittest.TestCase):
             (_gsmtap_hdr(GSMTAP_SIM_ATR), b'\x3b\x04', 0, 0),
             (_gsmtap_hdr(GSMTAP_SIM_APDU), b'\x80\xf2\x00\x00\x00', 0, 1),
         ])
-        parsed = [(t, payload) for _ts, t, payload in parse_pcapng(data)]
+        parsed = [(t, payload) for _ts, t, payload, _f in parse_pcapng(data)]
         self.assertEqual(parsed, [
             ('atr', b'\x3b\x04'),
             ('tpdu', b'\x80\xf2\x00\x00\x00'),
@@ -212,7 +224,7 @@ class TestImportEndpoint(unittest.TestCase):
 class TestVersion(unittest.TestCase):
     def test_package_version(self):
         from simtrace2_pysniff import __version__
-        self.assertEqual(__version__, '1.21.0')
+        self.assertEqual(__version__, '1.22.0')
 
     def test_status_includes_version(self):
         import os