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");

src/shared/file_receive.c

@@ -569,6 +569,13 @@ static FileSaveResult file_save_write_device(const char* root_directory, const F
 
 FileSaveResult file_save_to_disk_full(const char* root_directory, const File* file,
                                       const Config* config) {
+  /* Central no-mutation guard: a server-contacting --dry-run (or a local batch
+     apply that somehow carries dry_run) must never touch the destination, no
+     matter which caller reached this primitive.  The per-caller guards remain,
+     but this is the last line of defense for every save path.  Report SKIPPED
+     so a --remove-source-files sender correctly keeps its source. */
+  if (config && config->dry_run)
+    return FILE_SAVE_SKIPPED;
   /* Backups are incompatible with ignore-existing: moving the entry first
      would make a concurrent no-replace commit overwrite its old name. */
   bool backup_enabled = config && config->backup && !config->ignore_existing;
@@ -2941,6 +2948,11 @@ bool manifest_delete_missing_args(const Config* config, DeleteManifest* manifest
 bool manifest_delete_all(const Config* config, DeleteManifest* manifest) {
   if (!config || !manifest)
     return false;
+  /* Central no-mutation guard: a dry-run never deletes.  No manifest is sent on
+     the dry-run path, but a hostile/buggy peer could; treat it as a no-op so
+     the receiver can never remove anything. */
+  if (config->dry_run)
+    return true;
   if (config->delete_missing_args && !manifest_delete_missing_args(config, manifest))
     return false;
   if (config->use_delete && !manifest_delete_extras(config, manifest))