Move the maintained gallery into a readable .ebox fixture and make ebox-playground own generic parsing, file mode, and public rendering. Keep the startup mode trigger deferred through the user's use-package configuration.
Verified with ebox-playground make check (6/6), etaf-playground make check (6/6), exact use-package startup harness, and fullscreen GUI capture at /tmp/ebox-playground-dsl-gallery.png.
Replace the bare fixture with a 720 px three-section Grid and Flex reference, add paired design documentation, and lock the compact-window width contract.\n\nVerified with make check and a single-window GUI capture at a 784 px body width; rendered content stayed at 720 px.
