test(engine): add missing coverage for settings and animation plugins
settings_plugin: tests for cycle_unlocked (wrap, advance, single-element,
unknown-current, empty), volume floor clamping, and O-key screen toggle.
animation_plugin: tests for anim_speed_to_secs mapping (Fast < Normal,
Instant = 0), toast auto-dismiss on expired timer, toast survival when
timer positive, InfoToastEvent spawning a ToastOverlay, and
SettingsChangedEvent updating EffectiveSlideDuration.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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