fix: address c-review for -R/--dirs/--mkpath row

B1: destination-root existence/creation mishandled two legitimate forms.
file_directory_exists_secure/file_ensure_directory_secure now normalize a
trailing-slash destination (so the last component is never an empty leaf)
and treat a destination equal to the already-open authorized root as present
(no spurious <root>/<basename> nested dir with --mkpath). Confinement and
O_NOFOLLOW probing are unchanged. Regression integration tests: existing
dest with trailing slash works with and without --mkpath; dest == authorized
root works and creates no stray nested dir.

W1: chunk serialize/deserialize round-trip unit test for a directory entry
mixed with a regular file, with and without metadata; --dirs coverage under
-s and -s -m.
W2: --list-only and --dry-run now print file_wire_path() so all outputs show
the -R transformed relative name, matching -i/--out-format.
W3: RSYNC_COMPAT -d/--dirs note: dirs are created immediately under
--delay-updates (only regular files are staged).
W4: --dirs generator flushes chunks by element count too, so a long
--files-from list of empty directories cannot exceed the per-chunk file cap.
N1: STATUS_MKDIR added to status_to_string.
N2: removed dead TestMkpath._transfer.
N3: removed redundant --no-implied-dirs OPTION_TABLE row.
N4: integration tests: --dirs --delete keeps the just-created empty dir (and
deletes extras); a listed dir colliding with a regular file at the dest fails
cleanly.

src/client/scanner.c

@@ -465,6 +465,11 @@ static int open_next_directory(DirectoryScanner* scanner) {
    listed regular files are transferred as files; nothing else is scanned, so
    no descent into a listed directory can happen. */
 
+/* Directory entries carry no payload, so the dirs generator also bounds every
+   chunk by element count; chunk_deserialize refuses more than this many files
+   per chunk (see MAX_FILES_PER_CHUNK in chunk.c). */
+#define DIRS_CHUNK_MAX_FILES 65536U
+
 /* Build the File for the transfer root directory itself (the `-d <dir>` and
  * "." cases). */
 static File* dirs_root_dir_file(DirectoryScanner* scanner) {
@@ -618,6 +623,11 @@ static Chunk* directory_scanner_next_dirs(DirectoryScanner* scanner) {
       return NULL;
     }
     scanner->dirs_batch_size += file->data ? file->data->size : 0;
+    /* Empty directory entries carry no bytes, so a large --dirs --files-from
+       list must also be bounded by element count (the chunk deserializer caps
+       the number of files per chunk). */
+    if (scanner->dirs_batch->size >= (int)DIRS_CHUNK_MAX_FILES)
+      return dirs_flush_batch(scanner);
   }
   return dirs_flush_batch(scanner);
 }