Build a lexical repository symbol index

Input maps virtual Python paths to source text. Parse without importing or executing. List class, function, and async-function definitions that occur directly in module, class, or function bodies, including nested definitions. Return path, dotted lexical name, one-based line, and kind; sort by path, line, name. This deliberately bounded index does not traverse definitions inside conditional/loop blocks or resolve runtime dispatch. Invalid syntax raises ValueError containing the path. All data remains in memory.