Fix dropped change flags + FIDI parsing

- capture.py: DirectSniffer/GsmtapListener yield (type, data, flags),
  CaptureManager passes flags to insert_message (change flags were
  previously dropped, leaving every change row with flags=0)
- protocol.py: add _parse_fidi (1-byte value) — FIDI was mis-mapped to
  _parse_change (4-byte flags), so FIDI messages were silently dropped
- database.py: pass flags into decode_sniff_msg
- decode.py: decode_change(flags) reads flags directly; decode_sniff_msg
  gains flags parameter; change messages no longer require non-empty data
- tests: fix FIDI test, add change/fidi decode tests (43 total)

simtrace2_pysniff/protocol.py

@@ -85,6 +85,12 @@ def _parse_change(payload):
     consumed = 4
     return consumed, flags, b''
 
+
+def _parse_fidi(payload):
+    if len(payload) < 1:
+        return None
+    return 1, 0, payload[:1]
+
 _SNIFF_DATA_FMT = '<IH'
 _SNIFF_DATA_HDR_SIZE = struct.calcsize(_SNIFF_DATA_FMT)  # 6 bytes
 
@@ -102,7 +108,7 @@ def _parse_data(payload):
 
 _PAYLOAD_PARSERS = {
     SIMTRACE_MSGT_SNIFF_CHANGE: _parse_change,
-    SIMTRACE_MSGT_SNIFF_FIDI:   _parse_change,
+    SIMTRACE_MSGT_SNIFF_FIDI:   _parse_fidi,
     SIMTRACE_MSGT_SNIFF_ATR:    _parse_data,
     SIMTRACE_MSGT_SNIFF_PPS:    _parse_data,
     SIMTRACE_MSGT_SNIFF_TPDU:   _parse_data,