Function generate_core_excerpts 

pub fn generate_core_excerpts(
    all_fragments: &[Fragment],
    core_ids: &FxHashSet<FragmentId>,
    hunks: &[DiffHunk],
) -> FxHashMap<FragmentId, Fragment>

Hunk-window stand-ins for core fragments, keyed by the core each replaces. Kept out of all_fragments on purpose: they must not become graph nodes or ordinary context candidates, only substitutes for their own core.