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)

.gitignore

@@ -6,3 +6,4 @@ dist/
 build/
 .venv/
 decode-errors.log
+auth-test.log