fix(p7-privilege): harden copy-as/super gates, own dirs/specials

- fake-super owner replay honors --no-super and an active --copy-as
- copy-as/identity ownership now applied to directories and special nodes
- reject copy_as_set && !use_metadata (receiver + client --no-preserve)
- daemon refuses --copy-as; add server-side --no-super operator veto
- implement identity_copy_as_refused/identity_copy_as_active
- reject copy-as ids that overflow int32; escape spec in log errors
- copy-as chown EPERM/EACCES logged at ERROR (still non-fatal)
- identity_wire_valid copy-as bounds; CLI help and RSYNC_COMPAT docs
- add unit tests and root-gated integration coverage

src/client/usage.c

@@ -190,6 +190,13 @@ void print_usage(void) {
   printf("                      Names resolve on the source machine; @N for numerics.\n");
   printf("                      (Metadata is enabled with --preserve; -M now means\n");
   printf("                      rsync's --remote-option.)\n");
+  printf("  --copy-as=USER[:GROUP]  Force every written entry (files, dirs, symlinks\n");
+  printf("                      and special nodes) to USER[:GROUP], resolved on the\n");
+  printf("                      source machine like --chown.  Requires a privileged\n");
+  printf("                      (root) receiver and implies --preserve; an\n");
+  printf("                      unprivileged receiver refuses the transfer.  Never\n");
+  printf("                      switches process credentials (safe-subset; see\n");
+  printf("                      RSYNC_COMPAT.md).  A daemon refuses it.\n");
   printf("  --chunk-size <n>    Chunk size in bytes (default: %d)\n", DEFAULT_CHUNK_SIZE);
   printf("  --source-dir <path> Source directory\n");
   printf("  --dest-dir <path>   Destination directory\n");