Booking stream gains :hold/:confirm/:release; fold tracks per-actor seat state
(:held/:confirmed). A held seat counts toward capacity so a pending payment
can't be oversold. ev/hold! (capacity-safe), ev/confirm!, ev/release!,
ev/seat-state. Holds race test mirrors the booking race. 144/144 green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>