feat(run): add backpressure for runtime output consumers

[GaspardKirira]

Summary

Prevent slow event consumers, renderers, or replay writers from blocking or
exhausting the runtime process.

Requirements

• Use a bounded event queue.
• Define behavior when consumers are slow.
• Preserve important lifecycle events.
• Avoid deadlocks between process readers and consumers.
• Support cancellation.
• Report dropped or coalesced output explicitly.

Acceptance criteria

• Slow consumers do not deadlock execution.
• Event queues remain bounded.
• Lifecycle events are not silently lost.
• Cancellation unblocks producers and consumers.
• Dropped output is reported.
• Stress tests cover high-volume output.