Save-Data mode

The Save-Data client hint / navigator.connection.saveData signal by which a user requests reduced data usage, letting a site drop heavy assets on metered or low-bandwidth connections.

grounded in: Commit 3f6d515 'self-improve: Drop the matrix/sky canvases on data-saver connections' and a63befb 'Drop the decorative CRT shader on metered connections' — a real Save-Data web-platform signal, parall