feat(codecs): per-codec level defaults, RSYNC_*_LIST auto, zlibx reclassify

- rsync 3.4.1 per-codec defaults (zstd 3, zlib/zlibx 6, lz4 level ignored)
  and per-codec clamping; explicit --zl still wins
- auto resolves via whitespace-separated RSYNC_COMPRESS_LIST /
  RSYNC_CHECKSUM_LIST (first supported wins; all-unknown exits 4)
- zlibx reclassified: FastSync's zlib stream already excludes matched data,
  so its tree/stdout/exit match zlib
- --compress/-z and --compress-choice flip to parity (113/12/32)

src/shared/config.c

@@ -70,6 +70,8 @@ static void config_set_defaults(Config* config) {
   config->ignore_missing_args = false;
   config->checksum_transfer_algo = CHECKSUM_ALGO_DEFAULT;
   config->cli_exit_code = 0;
+  config->compression_level_set = false;
+  config->checksum_choice_set = false;
   config->filters = NULL;
   config->files_from = NULL;
   config->files_from_set = NULL;