Function execute 

pub fn execute(
    plan: &mut InstallPlan,
    extracted_dir: &Path,
    store_mod_dir: &Path,
) -> InstallerResult<Vec<StagedFile>>

Execute plan, staging files from extracted_dir into store_mod_dir. On success, mutates plan.staged_files with the final manifest and returns it by value for the caller to persist.

extracted_dir is the temp directory the archive was unzipped into. store_mod_dir is the canonical per-mod directory in the store (the caller decides the naming, typically {domain}_{mod_id}_{file_id}).