Seeded timing and velocity jitter over note attacks.
Timing jitter is a forward-only delay in [minimum_timing_samples, timing_samples]. A kernel on the audio thread cannot emit an event earlier than it arrived, so a symmetric jitter would only be expressible by delaying the whole stream; a forward delay keeps the kernel causal and keeps latency equal to the declared bound rather than hiding it.
Both jitter draws come from timebase::coordinate_random over the event's absolute sample position, so the same authored note receives the same jitter no matter how the caller partitions its blocks.