fix(fs): rsync push iconv direction; gate empty-dir emission; reclassify --temp-dir

- --iconv now matches rsync's push direction: the destination charset is the
  client spec's REMOTE half, so a default receiver writes wire names verbatim;
  a server's own --iconv LOCAL overrides it (daemon charset analog). Updated
  unit + integration tests and added a default-server differential gate case.
- Empty-directory emission is gated behind a new ScannerOptions.emit_empty_dirs
  set only by the real sender, so low-level scanner helpers keep the historical
  file-only list.
- --temp-dir reclassified to Divergent: relative dirs match rsync exactly
  (resolved under the destination), but an absolute path is deliberately
  rejected by the confined receiver; differential test added.
- Docs/tally: 109 Parity / 22 Caveat / 25 Divergent.

src/client/client_send.c

@@ -412,6 +412,9 @@ static bool prepare_scanner(const Config* config, int num_threads, PreparedScann
   options->exclude_per_dir_filter_files = config->per_dir_filter_count >= 2;
   options->dirs = config->dirs;
   options->relative = config->relative;
+  /* A real recursive transfer recreates empty source directories (rsync
+     parity); low-level scanner users leave this off. */
+  options->emit_empty_dirs = true;
   /* --no-implied-dirs only has meaning with -R (rsync): without it the option
      is a documented no-op, so the scanner must not suppress directory
      metadata. */