feat(p7-super): implement --super/--no-super safe-subset privilege gate (protocol 2.18.0)

Add the receiver-side --super / --no-super tri-state (Config->super_mode)
under the safe-subset + clear-refusal privilege model: FastSync never
elevates privileges, it only permits super-user attempts that are already
confined fd-relative below the authorized receive root.

- identity: privilege_super_permitted() gate (OFF=false, ON=true, AUTO follows
  geteuid()==0); identity_apply_ownership/_link become no-ops when not
  permitted; --super with no explicit identity policy implies raw numeric-id
  preservation (explicit usermap/groupmap/chown/numeric-ids still win); warn
  exactly once when --super is requested by a non-root receiver.
- file_receive: gate char/block device-node creation on the gate; FIFO/socket
  handling is unchanged.
- wire: trailing super_mode int after the --iconv spec, validated 0..2 in
  receive_privilege_options and validate_received_config; PROTOCOL_VERSION
  2.17.0 -> 2.18.0; version-sensitive tests and docs updated.
- CLI: --super/--no-super parsed explicitly before the generic --no-* branch
  (malformed --super=x rejected); usage text added.
- tests: config wire round-trip + invalid-value rejection, privilege-gate mode
  unit test, CLI parse test, integration transfer + root-gated ownership
  suppression/appliance tests.
- docs: RSYNC_COMPAT --super row + Wave E note, protocol mentions, README.

README.md

@@ -507,7 +507,7 @@ defaults to the current directory. |
 
 ## Protocol and Security
 
-FastSync protocol version `2.5.0` is shared by the client and server. The
+FastSync protocol version `2.18.0` is shared by the client and server. The
 current protocol is sender-driven and includes configuration negotiation,
 including the maximum allocation limit, incremental checks, checksums,
 manifests, keep-alives, abort handling, per-file remove-source results, and