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.

README.md

@@ -119,7 +119,11 @@ passive SIM sniffer that emits the same GSMTAP-SIM format on UDP 4729. Its
 custom sub-types for RST/VCC line events (`0x10`/`0x11`, added for
 compatibility with that project) are decoded as `RESET ASSERTED/DE-ASSERTED`
 and `VCC ON/OFF` timeline markers, and a reset-assert or power-removal
-correctly invalidates the file-selection tracking.
+correctly invalidates the file-selection tracking. GSMTAP packets carrying
+the `GSMTAP_FLAG_BAD_FCS` desync flag (set by that decoder on mis-framed
+exchanges) are surfaced as a flagged/desynced entry in the capture timeline,
+so such artefacts are easy to spot when reviewing a capture rather than
+silently altering the decode.
 
 Server options: