Function read_saved_options 

pub fn read_saved_options(grammar_json: &str) -> Result<SavedOptions, LoadError>

Read the compile-invariant options a save recorded in data.options. A separate pass over the document, like [read_lexer_conf] — from_json_str needs maybe_placeholders BEFORE load_grammar runs, because it shapes the rules the loader builds.

A missing data/options block is not an error (older saves and the hand-authored fixtures omit it): it yields an all-None SavedOptions. A present key of the wrong JSON type IS an error — silently ignoring it would reintroduce the divergence this exists to close.