feat(sender): --ignore-missing-args / --delete-missing-args CLI and files-from preflight

New flags parse onto the config; --delete-missing-args implies
--ignore-missing-args (order-independent) and does NOT imply --delete (rsync:
independent of other delete processing). The files-from preflight now classifies
listed-but-missing entries instead of hard-failing: under the flags each is
skipped (logged + counted, never silent) and the run succeeds for the rest,
including the all-missing case; an empty list stays a hard error. Under
--delete-missing-args the missing entries' destination mirrors (bare relative
path with -R, full source mirror otherwise) ride the manifest's third section in
both the single-threaded and -m senders; --dirs listed-but-missing entries are
skipped in the scanner.

src/shared/multiprocessing.c

@@ -27,6 +27,7 @@ PipelineContextSender* pipeline_context_sender_create(Config* config, Queue* que
   context->loader_done = false;
   context->manifest = NULL;
   context->excluded_paths = NULL;
+  context->missing_args = NULL;
   context->scan_had_io_error = false;
   context->remove_source_files = NULL;
   context->early_delete = false;
@@ -86,6 +87,8 @@ void pipeline_context_sender_destroy(PipelineContextSender* context) {
   }
   if (context->excluded_paths)
     array_list_delete(context->excluded_paths);
+  if (context->missing_args)
+    array_list_delete(context->missing_args);
   if (context->remove_source_files)
     array_list_delete(context->remove_source_files);
   config_delete(context->config);