fix(receive): enforce dry-run no-mutation centrally

--dry-run --read-batch=FILE still wrote to the destination because
batch_read_apply -> file_save_to_disk_full bypassed the per-caller
!dry_run guards.  Guard file_save_to_disk_full and manifest_delete_all
directly (return SKIPPED/no-op) so every save/delete path is mutation-free
in dry-run, and keep the per-caller guards.  Reject --dry-run combined with
--read-batch/--only-write-batch at CLI validation with a clear error (a
dry-run of a local batch apply is not meaningful).

src/client/client_validation.c

@@ -22,6 +22,17 @@ bool validate_config(const Config* config) {
                 "--write-batch, --only-write-batch, and --read-batch are mutually exclusive");
     return false;
   }
+  /* A dry-run of a local batch apply is not meaningful: --read-batch bypasses
+     the client-side scan/server decision entirely, so dry-run would have no
+     wire state to report (and must not be used as a mutation escape hatch).
+     --only-write-batch likewise never contacts a receiver.  Reject both up
+     front instead of silently ignoring --dry-run. */
+  if (config->dry_run && (read_batch || only_write_batch)) {
+    log_message(LOG_LEVEL_ERROR,
+                "--dry-run cannot be combined with --read-batch or --only-write-batch; "
+                "a dry-run of a local batch apply is not meaningful");
+    return false;
+  }
   if (read_batch) {
     if (!config->receive_root_directory) {
       log_message(LOG_LEVEL_ERROR, "--read-batch requires a destination directory");