fix(delete): guard the per-directory delete commit against dry-run

delete_plan_session_commit() lacked the central no-mutation guard that
manifest_delete_all() has, so a server-contacting -n run (or a hostile plan
frame) could still remove --delete-missing-args mirrors on the per-directory
timing path.  Return DELETE_COMMIT_OK immediately when the session is a
dry-run, and gate the receiver/server commit call sites too.  Add a unit test
that streams a plan naming an existing destination file and asserts it
survives.

src/server/server.c

@@ -970,7 +970,11 @@ void handler(int file_descriptor) {
          arrived; with the disk writer drained, commit the deferred removals.
          --delete-during already applied its plans on the receive thread. */
       if (context->deferred_plans) {
-        DeleteCommitResult deletion = delete_plan_session_commit(context->deferred_plans, config);
+        /* Defence in depth (the enclosing block already excludes dry-run): a
+           -n run never commits a deletion. */
+        DeleteCommitResult deletion = config->dry_run ? DELETE_COMMIT_OK
+                                                      : delete_plan_session_commit(
+                                                            context->deferred_plans, config);
         if (deletion == DELETE_COMMIT_ERROR) {
           transfer_ok = false;
         } else if (deletion == DELETE_COMMIT_LIMIT_REACHED) {