Function apply_pack 

async fn apply_pack(scheduler: &Scheduler, pack: &ContentPack) -> MergeSummary

Merge pack into the active profile and persist. Mirrors the update_settings write sequence: merge into a clone, rebuild the #[serde(skip)] derived caches (the break fire path reads derived.*_hints_resolved, not the raw pools — without this an imported idea stays invisible until the next settings change), store, then upsert the active profile (pushing it if somehow absent, like update_settings) and persist. Split out so it’s unit-testable without a WebviewWindow.