BrowserCore now owns an AppearanceSettings, exposes it through
BrowserSnapshot.appearance, and offers set_wallpaper_theme,
set_theme_mode, set_reduce_motion, reset_appearance mutators. Defaults
match the domain defaults (Dawn / System / motion-on) so existing
behaviour is unchanged for callers who don't touch appearance.
Integration tests cover initial snapshot defaults, mutation persistence
across snapshots, and reset behaviour.