Function plan_migration 

pub fn plan_migration(
    old_entries: &[Map<String, Value>],
    new_entries: &[Map<String, Value>],
    stems_dir: &Path,
    strict: bool,
) -> Vec<Action>

Compare old and new entries and produce one action per new entry.

Matching is two-phase: exact on (text, speaker), then a text-only fallback for dialogue so a speaker reassignment is reported as such rather than as a brand-new line.