Pushed after configure has completed, when the deferred file walk and
language detection produce warnings (missing formatter/checker/LSP binaries,
or “project too large” file-count exceeded). The walk runs in a background
thread so configure itself returns in <100 ms even on huge directories
(e.g. user’s $HOME). When the walk finishes, AFT pushes one frame with
the merged warnings — the plugin delivers them through the same path as
the synchronous warnings that configure used to return.