Every ckeditor5-ui button variant × content (just label, label & icon,
just icon) × state (default, hover, disabled, active/on, active hover), using only UI
classes — no feature-specific ones.
The Hover and Active hover columns are simulated with a
ck-force-hover class (the manual harness has no pointer).
Focus is a live-only state — Tab to a button to check it
(e.g. the Surface row / ck-button_surface also lifts to the
surface color on focus).
The checkerboard cell background makes transparent vs. filled button backgrounds easy to spot.