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/usage.c

@@ -90,8 +90,8 @@ void print_usage(void) {
   printf("                      entry's destination mirror receiver-side.  Independent of\n");
   printf("                      --delete (it does not imply --delete; a non-empty directory\n");
   printf("                      mirror is removed only with --force or --delete)\n");
-  printf("  -m, --prune-empty-dirs  Do not transfer empty directory entries (--dirs mode);\n");
-  printf("                      recursive transfers never send empty dirs\n");
+  printf("  -m, --prune-empty-dirs  Do not create empty directories (a recursive transfer\n");
+  printf("                      otherwise recreates them, like rsync)\n");
   printf("  Note: each timing flag implies --delete.  Combining a timing flag with\n");
   printf("  --no-delete (in either order) is rejected as a config error.\n");
   printf("  --ignore-existing  Skip files that already exist on receiver\n");