Function build 

pub fn build(
    builtins: Vec<BuiltinPolicy>,
    startup: &HashMap<String, ParsedPolicy>,
    operator: &HashMap<String, ParsedPolicy>,
    languages: &[&dyn Language],
    last_good: &LastGoodPolicies,
) -> (PolicyTree, LoadReport)

Build the full PolicyTree from every source.

builtins is source 1 (crate::builtin::builtin_policies). startup is source 2’s already-parsed records supplied at startup, compiled with no last-good-revision fallback: it is immutable for the run, so there is never a second revision to have fallen back from. operator is source 3’s current view – a snapshot taken once at the start of this call – compiled with last-good-revision fallback via last_good, which must be the same LastGoodPolicies passed on every call across this deployment’s lifetime, or there is nothing for a bad revision to fall back to. languages are every Language this build carries, used to compile startup and operator predicates (builtins need none: they carry already-constructed evaluators).