fn build_installed_manifest(gen_dir: &Path) -> Result<Value>Expand description
Builds the “installed” manifest via the exact same code path regen uses
to write methods.json (load_combined_defs + build_methods_manifest),
then converts it to a Value so it can be diffed with the same
section-shaped logic as the vendored JSON file - no separate parsing path
to drift out of sync with the real one.