Move ephemeral rendering, initial buffer mounts, declarative commits, and opaque handle updates onto retained TP surfaces while keeping Ebox layout planning and runtime indexes transactionally synchronized. Verified with: make check EMACS=/Applications/Emacs.app/Contents/MacOS/Emacs
Split the verified renderer, layout engine, Grid support, native boundary, tests, examples, and paired documentation into the independent Ebox repository. Keep ETAF and application concerns outside this package.
