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/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