Japanese was never dispatched through the Lang table: dotrans and
transstr both special-cased it into transjp, which edited Im in place
and wrote commits through a side argument, while transko popped and
cleared a pre that every caller overwrote anyway, and Vietnamese kept
its key history in dotrans. Emit now carries the new raw state next to
the new pre; transjp, transko and transvi are pure functions of (Im,
key), and dotrans and transstr have a single path. Vietnamese history
bookkeeping lives in vi.c, where the full-history flush no longer
resets the caret as a side effect; istone was toneidx() >= 0.
