Function plan_index 

pub fn plan_index(
    store: &IndexStore,
    project_root: &Path,
    extensions: &[&str],
) -> Result<(Vec<FileEntry>, usize)>

Determine which files need (re)indexing by comparing BLAKE3 hashes.

Returns (files_to_index, cached_count).

Errors

Returns an error if walking the source tree fails.