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.

tests/integration/test_differential_parity.py

@@ -233,6 +233,11 @@ _CASES = [
            ["-a", "--iconv=ISO-8859-1,UTF-8"],
            server_args=("--allow-super", "--iconv=UTF-8"),
            ref="--iconv conversion (receiver declares its own charset)"),
+    # rsync's spec is LOCAL,REMOTE and the destination end's charset is REMOTE
+    # on a push, so a default server writes the wire (UTF-8) names verbatim.
+    H.Case("iconv_default_server", "iconv",
+           ["-a", "--iconv=ISO-8859-1,UTF-8"],
+           ref="--iconv push direction (default receiver charset = REMOTE)"),
 
     # --- partial ----------------------------------------------------------
     H.Case("partial_complete", "basic", ["-a", "--partial"], ref="--partial"),