Function get_locator_from_start 

pub fn get_locator_from_start() -> Locator

get_locator’s accurate-start sibling (§1, docs/performance/SOURCE_PROVENANCE.md): same mouthstack walk, but reads the mouth’s get_locator_from_start (from = the last token’s captured start) instead of the heuristic from. Used for the construct-START snapshot at constructor digest under --source-map.