Formalize the trial-boot / commit / rollback capabilities as traits

[rusty1968]

fwmanager's BootControl doc already sketches this flow (set_trial → release → await_boot → commit),

pub enum BootProgress { Booted, Failed, Timeout }

pub trait BootMonitor {
    type Error: core::error::Error;
    fn await_boot(&mut self, window: core::time::Duration)
        -> Result<BootProgress, Self::Error>;
}

pub trait SlotControl {
    type Error: core::error::Error;
    fn set_trial(&mut self, slot: Slot) -> Result<(), Self::Error>;
    fn commit(&mut self, slot: Slot)    -> Result<(), Self::Error>;
    fn rollback(&mut self)              -> Result<(), Self::Error>;
}


fn trial_boot<D, M, S>(dev: &mut D, monitor: &mut M, store: &mut S, new_slot: Slot)
    -> Result<(), Error>
where
    D: BootControl,
    M: BootMonitor,
    S: SlotStore,
{
    dev.hold_in_reset()?;
    store.set_trial(new_slot)?;                 // tentative — not yet committed
    dev.release()?;                             // boot the trial image

    match monitor.await_boot(window)? {         // Ok(progress); Err only = couldn't watch
        BootProgress::Booted  => store.commit(new_slot)?,   // observed good => active
        BootProgress::Failed  => store.rollback()?,         // bad image => previous slot
        BootProgress::Timeout => store.rollback()?,         // went quiet => previous slot
    }
    Ok(())
}