instrumentation for the ~1,300 host CPU cliff: - relay_persist_order_spins_total: spin iterations on the ordering lock - relay_broadcast_queue_full_total: spin iterations on full broadcast queue - relay_broadcast_queue_depth_hwm: high-water mark of queue depth - relay_broadcast_no_consumers_total: frames that skipped SharedFrame alloc zero-consumer fast path: when no consumers are connected, broadcast() returns after history.push() without allocating SharedFrame or taking consumers_mutex. saves one heap alloc + one mutex per frame. also includes the cursor coalesce fix (CursorMap) and slot reuse (free list with unregister on subscriber exit) from previous commits. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>