feat: implement -R/--relative, --no-implied-dirs, --dirs/-d, --mkpath

RSYNC_COMPAT Phase-2 row M: path-list construction and destination
directory creation while preserving traversal safety.

- -R/--relative with --files-from: transmit each listed entry under its
  bare relative destination path (no source-root mirror). Files keep an
  absolute local read path plus a separate wire/dest path (File.send_path);
  manifest, incremental quick-check and change output follow the wire path,
  so --delete and --remove-source-files stay consistent. -R without
  --files-from is unchanged (full mirror).
- --no-implied-dirs: client-only, only meaningful with -R + --files-from.
  A listed file whose parent dir is not itself (or via an ancestor)
  explicitly listed cannot be placed; the run fails up front with a clear
  error. No effect otherwise.
- --dirs/-d + --old-dirs/--old-d aliases: -d <dir> transmits the source
  root as an explicit empty directory entry (STATUS_MKDIR frame); with
  --files-from listed dirs are created empty and listed files transferred,
  never descending. Works single-threaded, -m (sequential scanner in the
  -m scan thread) and chunk-serialization (per-file type marker).
  Directory entries appear in the delete manifest.
- --mkpath: new wire bool; server creates the destination root (and missing
  leading components under its authorized root) at connection start. A
  missing destination root is now rejected by default.
- Protocol bumped to 2.7.0 (mkpath wire field + STATUS_MKDIR + chunk type
  marker). All receive paths funnel through file_save_to_disk_full which
  creates directories via the secure confined mkdir engine; dir entries are
  excluded from --remove-source-files outcome acknowledgements on both ends.
- Unit coverage: CLI parse (relative/dirs aliases/mkpath/no-implied-dirs),
  config round-trip (relative + mkpath), scanner --dirs non-recursion and
  -R send_path (sequential + parallel), receiver dir-entry save.
- Integration coverage: TestRelativeFilesFrom, TestNoImpliedDirs, TestDirs,
  TestMkpath (single and -m).
- RSYNC_COMPAT: 4 rows move to Implemented (Summary 67/3/5/1/71 = 147).

src/client/client_cli.c

@@ -425,8 +425,12 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--ignore-existing", NULL, OPT_FLAG, offsetof(Config, ignore_existing)},
     {"--delay-updates", NULL, OPT_FLAG, offsetof(Config, delay_updates)},
     {"--chmod", NULL, OPT_STRING, offsetof(Config, chmod_spec)},
-    {"--dirs", "--old-dirs", OPT_UNSUPPORTED, 0},
-    {"--old-d", NULL, OPT_UNSUPPORTED, 0},
+    {"--dirs", "-d", OPT_FLAG, offsetof(Config, dirs)},
+    {"--old-dirs", NULL, OPT_FLAG, offsetof(Config, dirs)},
+    {"--old-d", NULL, OPT_FLAG, offsetof(Config, dirs)},
+    {"--relative", "-R", OPT_FLAG, offsetof(Config, relative)},
+    {"--no-implied-dirs", NULL, OPT_FLAG, offsetof(Config, no_implied_dirs)},
+    {"--mkpath", NULL, OPT_FLAG, offsetof(Config, mkpath)},
     {"--delete-during", "--del", OPT_UNSUPPORTED, 0},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
@@ -576,9 +580,7 @@ static int apply_table_option(Config* config, const OptionEntry* entry, const ch
     return 0;
   }
   case OPT_UNSUPPORTED: {
-    const char* reason = "directory-only transfer is not implemented";
-    if (strcmp(entry->name, "--delete-during") == 0)
-      reason = "delete-during is not implemented";
+    const char* reason = "delete-during is not implemented";
     log_message(LOG_LEVEL_ERROR, "%s: %s; refusing to ignore option", option_name, reason);
     return -1;
   }
@@ -611,6 +613,13 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
       config->show_progress = true;
       continue;
     }
+    /* "--no-implied-dirs" is a real rsync option name, not a negation of
+     * "--implied-dirs", so it must be handled before the generic --no-*
+     * negation branch. */
+    if (strcmp(argv[i], "--no-implied-dirs") == 0) {
+      config->no_implied_dirs = true;
+      continue;
+    }
     if (strncmp(argv[i], "--no-", strlen("--no-")) == 0) {
       if (apply_negation(config, argv[i]) != 0)
         return -1;