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/server/capture.py

@@ -34,10 +34,10 @@ class GsmtapListener:
 
     def iter_messages(self):
         while self._running:
-            sub_type, data = self._receiver.read_packet()
+            sub_type, data, flags = self._receiver.read_packet()
             if sub_type is None:
                 continue
-            yield gsmtap_msg_type(sub_type), data, 0
+            yield gsmtap_msg_type(sub_type), data, flags
 
 
 class DirectSniffer:

simtrace2_pysniff/server/decode.py

@@ -4030,6 +4030,19 @@ def decode_data_flags(flags):
     return names or None
 
 
+def gsmtap_flag_names(flags):
+    """Decode the GSMTAP header ``res`` per-packet flag bits into a list of
+    human-readable names (or None).
+
+    GSMTAP_FLAG_BAD_FCS is set by the sigrok-iso7816-stream decoder on a
+    desynced TPDU (mis-framed exchange / implausible status word).
+    """
+    names = []
+    if flags & 0x01:
+        names.append('bad FCS (desynced)')
+    return names or None
+
+
 def decode_fidi(raw_data):
     """Decode a SIMtrace2 sniff_fidi payload (1-byte Fi/Di)."""
     if len(raw_data) < 1:
@@ -4274,27 +4287,31 @@ def decode_sniff_msg(raw_data, msg_type, flags=0, prev=None):
     if msg_type == 'tpdu' and raw_data:
         result = decode_message(raw_data, prev=prev)
         if result is not None:
-            errs = decode_data_flags(flags)
+            errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
             if errs:
                 result['errors'] = errs
         return result
     if msg_type == 'change':
         return decode_change(flags)
     if msg_type in ('rst', 'vcc'):
-        return decode_line_event(raw_data, msg_type)
+        result = decode_line_event(raw_data, msg_type)
+        errs = gsmtap_flag_names(flags)
+        if errs:
+            result['errors'] = errs
+        return result
     if msg_type == 'fidi' and raw_data:
         return decode_fidi(raw_data)
     if msg_type == 'atr':
         result = _decode_atr(raw_data)
         if result is not None:
-            errs = decode_data_flags(flags)
+            errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
             if errs:
                 result['errors'] = errs
         return result
     if msg_type == 'pps':
         result = _decode_pps(raw_data)
         if result is not None:
-            errs = decode_data_flags(flags)
+            errs = (decode_data_flags(flags) or []) + (gsmtap_flag_names(flags) or [])
             if errs:
                 result['errors'] = errs
         return result

simtrace2_pysniff/server/server.py

@@ -331,9 +331,9 @@ class RequestHandler(BaseHTTPRequestHandler):
         first_ts = packets[0][0]
         last_ts = packets[-1][0]
         rows = []
-        for ts, msg_type, payload in packets:
+        for ts, msg_type, payload, flags in packets:
             elapsed = round(ts - first_ts, 6) if ts is not None else 0.0
-            rows.append((max(0.0, elapsed), msg_type, payload, 0))
+            rows.append((max(0.0, elapsed), msg_type, payload, flags))
         self.db.insert_messages(session_id, rows)
         # Use the packet timestamps as the session window when they are real
         # (classic pcap / pcapng EPB).  pcapng SPB-only files have synthetic