Function apply_pattern 

pub(crate) fn apply_pattern<G, Y>(
    pattern: LedPattern,
    tick_high: bool,
    led_green: &mut G,
    led_yellow: &mut Y,
) -> Duration
where
    G: Led,
    Y: Led,

Apply one tick of pattern to the LEDs and return the duration to wait before the next tick.

tick_high alternates false/true each iteration. For static patterns (solid, all-off) the value is ignored.

Generic over [Led] so the firmware task and host-side unit tests can share the same code path.