Function fixed_break_due 

fn fixed_break_due(kind: BreakKind, s: &Settings, now_min: u32) -> bool

Pure decision: is a fixed-time break for kind due at now_min?

Reads the pre-parsed minutes from s.derived (resolved once at settings load/update) instead of re-parsing the "HH:MM" strings, and gates on the kind being enabled and in a fixed-firing schedule mode. Sleep has no fixed-time schedule and is always false.