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/scanner.c

@@ -1399,8 +1399,9 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
     if (entry == NULL) {
       /* The directory is exhausted: if nothing was transferred or descended
          from it, recreate it at the destination as an explicit entry. */
-      if (!scanner->current_dir_produced && !scanner->options.prune_empty_dirs &&
-          !scanner->options.list_dirs && scanner->options.file_list == NULL) {
+      if (scanner->options.emit_empty_dirs && !scanner->current_dir_produced &&
+          !scanner->options.prune_empty_dirs && !scanner->options.list_dirs &&
+          scanner->options.file_list == NULL) {
         if (!scanner_emit_empty_dir(scanner, chunk_data))
           scanner->failed = true;
       }