feat: event_status_reverted event kind + projector handler (T114.2)

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the post-superseded state.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.

chat/state/events.py

@@ -67,6 +67,29 @@ def _apply_event_expired(conn: Connection, e: Event) -> None:
     )
 
 
+@on("event_status_reverted")
+def _apply_event_status_reverted(conn: Connection, e: Event) -> None:
+    """T114.2: Revert an event row's status to ``prior_status``.
+
+    Emitted by ``regenerate_assistant_turn`` when a superseded turn had
+    triggered a lifecycle transition (event_started / event_completed /
+    event_cancelled). The rollback step needs an inverse projection that
+    sets the row's status back to whatever it was *before* the now-
+    superseded transition fired.
+
+    Unlike the forward transitions (which guard against terminal-status
+    overwrites) this handler is unconditional — the entire purpose is to
+    reverse a transition, including reverting from a terminal status
+    (completed/cancelled) back to a non-terminal one.
+    """
+    p = e.payload
+    conn.execute(
+        "UPDATE events SET status = ?, updated_at = datetime('now') "
+        "WHERE event_id = ?",
+        (p["prior_status"], p["event_id"]),
+    )
+
+
 def get_event(conn: Connection, event_id: str) -> dict | None:
     row = conn.execute(
         "SELECT event_id, chat_id, kind, status, props_json, planned_for, "