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.h

@@ -419,6 +419,12 @@ typedef struct Config {
    * for an unsupported checksum/compress algorithm) so main() can mirror it. */
   int checksum_transfer_algo;
   int cli_exit_code;
+  /* Client-only "the user explicitly chose" bits.  They let the per-codec
+   * default level / checksum list be applied only when the corresponding
+   * rsync option was omitted (an explicit --compress-level / --checksum-choice
+   * always wins).  Never serialized. */
+  bool compression_level_set;
+  bool checksum_choice_set;
 
   // Issue #129: Advanced file selection. These fields are CLIENT-ONLY: they are
   // never serialized to the wire (the receiver must not learn them).