test(dry-run): strengthen no-mutation coverage and refresh docs

Extend _snapshot_tree to record mode, inode, xattrs, directories and
special nodes, and add coverage proving a server-contacting --dry-run
leaves the destination structurally identical for --delay-updates,
--backup, symlinks, hardlinks, FIFOs, and daemon modules (including a
read-only module).  Add a regression test for the --read-batch --dry-run
refusal and for a missing/non-directory receive root failing a dry-run
exactly like a real run.

Fix stale version comments (2.20.0/633 -> 2.21.0/637) and RSYNC_COMPAT's
current --protocol value, and add a unit assertion that
--server-port/--port (and --server-host) set the dry-run routing bit.

tests/test_config.c

@@ -2399,7 +2399,7 @@ static void golden_config_populate(Config* c) {
   c->modify_window = 3;
   c->compress_choice = str_dup("zstd");
   /* "u=rwx,go=rx" is the same 11 bytes as the original "u=rwX,go=rX" (so the
-   * frame stays 633 bytes) but X is not in FastSync's chmod grammar, and the
+   * frame stays 637 bytes) but X is not in FastSync's chmod grammar, and the
    * receive-side golden validates the frame. */
   c->chmod_spec = str_dup("u=rwx,go=rx");
   c->skip_compress_set = true;
@@ -2531,7 +2531,7 @@ static unsigned long long capture_wire_hash(const Config* cfg, size_t* out_len)
   return h;
 }
 
-/* Byte-for-byte wire compatibility guard (protocol 2.20.0).  The expected hash
+/* Byte-for-byte wire compatibility guard (protocol 2.21.0).  The expected hash
  * pins the pre-X-macro byte stream; the refactor MUST NOT change it. */
 static void test_config_wire_golden() {
   if (is_running_under_valgrind())