Implements storyInit/storyGetFlagByName/storyGetFlagValue/etc, moves flag
storage into saveslot_t (persisted keyed by name so save files survive
flag reordering), and adds STORY_SET_FLAG/STORY_WAIT_FLAG cutscene items.
Also removes the now-superseded storyflag.c/h stub.
Co-Authored-By: Claude Sonnet 5 <[email protected]>