Implement two new C API functions to retrieve list metadata and values from Wren slots: wrenGetSlotListSize returns the element count of a list in a slot, and wrenGetSlotListValue returns a captured WrenValue pointer for a specific index. Add corresponding foreign method bindings in test/api/slots.c and test cases in test/api/slots.wren that verify size retrieval and indexed value access on a three-element list. Also add Damien Radtke to AUTHORS.
