WhiffStats.beaten_to_ball_count stays at 0 since beaten-to-ball moved to its own detector #284

Since #280, whiff detection no longer emits WhiffEventKind::BeatenToBall,
and the detector added in #278 only produces BeatenToBallEvents. Nothing
accumulates those events, so WhiffStats.beaten_to_ball_count is always 0
for new replays, and there's no other per-player beaten-to-ball count.

Is that intended? We show the count as a per-player stat, so it would help
to know which direction you have in mind:

1. The whiff accumulator counts the detector's events into
   beaten_to_ball_count. That's what we do in our fork for now:
   wilbo@5922cd1
2. A dedicated beaten-to-ball stats module, retiring the field on WhiffStats.
3. Beaten-to-ball stays event-only.

Happy to open a PR for whichever you prefer.