Adds the inverse projection used by T114.3's regenerate rollback. The
new ``event_status_reverted`` event kind carries
``{event_id, prior_status}`` and the handler unconditionally sets
``events.status = prior_status`` for the row.
Unlike the forward transitions (event_started / event_completed /
event_cancelled), this handler does NOT guard against terminal
statuses — its entire purpose is to reverse a transition, including
walking back from a terminal status to a non-terminal one. Without
that, rolling back an event_completed (status='completed' is terminal
for the forward handlers) would silently no-op and leave the row in
the post-superseded state.
The handler registers via the existing ``@on(kind)`` decorator pattern
in chat/eventlog/projector.py, so future replays of an event_log that
contains event_status_reverted rows pick it up automatically.
Test exercises completed→active, active→planned, and cancelled→active
round-trips.
Joseph Doherty