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.

simtrace2_pysniff/gsmtap.py

@@ -19,6 +19,10 @@ GSMTAP_SIM_PPS_RSP = 0x03
 GSMTAP_SIM_RST_EVENT = 0x10
 GSMTAP_SIM_VCC_EVENT = 0x11
 
+# Per-packet flags carried in the GSMTAP header `res` byte (libosmocore
+# gsmtap.h).  sigrok-iso7816-stream sets BAD_FCS on a desynced TPDU.
+GSMTAP_FLAG_BAD_FCS = 0x01
+
 GSMTAP_UDP_PORT = 4729
 
 _GSMTAP_HDR_FMT = '!BBBBHBBIBBBB'  # 16 bytes, big-endian
@@ -79,27 +83,29 @@ class GsmtapReceiver:
     def read_packet(self):
         """Block until a GSMTAP-SIM packet arrives.
 
-        Returns ``(sub_type, data)`` on success or ``(None, None)``
-        on timeout.  *sub_type* is ``GSMTAP_SIM_ATR`` or ``GSMTAP_SIM_APDU``.
+        Returns ``(sub_type, data, flags)`` on success or ``(None, None, 0)``
+        on timeout.  *sub_type* is ``GSMTAP_SIM_ATR``/``GSMTAP_SIM_APDU`` or
+        a custom sub-type; *flags* are the per-packet bits from the GSMTAP
+        header ``res`` byte (see ``GSMTAP_FLAG_*``).
         Raises ``ValueError`` for non-SIM or unknown GSMTAP packets.
         """
         try:
             packet, addr = self._sock.recvfrom(65536)
         except (socket.timeout, OSError):
-            return None, None
+            return None, None, 0
 
         if len(packet) < _GSMTAP_HDR_SIZE:
             raise ValueError(f'GSMTAP packet too short: {len(packet)} bytes')
 
         (version, hdr_len, pkt_type, _timeslot, _arfcn, _noise, _signal,
-         _frame, sub_type, _antenna, sub_slot, _res) = \
+         _frame, sub_type, _antenna, sub_slot, res) = \
             struct.unpack(_GSMTAP_HDR_FMT, packet[:_GSMTAP_HDR_SIZE])
 
         if pkt_type != GSMTAP_TYPE_SIM:
             raise ValueError(f'Not a GSMTAP SIM packet (type={pkt_type})')
 
         data = packet[_GSMTAP_HDR_SIZE:]
-        return sub_type, data
+        return sub_type, data, res
 
     def close(self):
         self._sock.close()