Concurrency and queues - sndbox docs

Concurrency exists at several independent layers.

Layer	Boundary
Workflow	`maxConcurrentNodes` bounds nodes eligible to run together.
Runner	Reported concurrency and current workload bound local claims.
Pool	Aggregate pool concurrency reserves assignment capacity.
Event queue	Visibility claims stop two workers from owning the same current delivery.
Scheduler	Advisory and row locking serialize occurrence emission.

Parallel graph branches still depend on typed inputs and control edges. Do not depend on incidental canvas order when two nodes are eligible together.

No available runner

Remote schedules and queue events apply the configured no-runner and expiry behavior. An offline, stale, draining, incompatible, disabled, or capacity-exhausted runner is visible in rejection diagnostics but cannot claim work.

Poison events

Repeated identical failures are bounded. After three identical failures or the configured attempt limit, the event moves to dead-letter state. Replay is a new audited delivery, not deletion of the failed evidence.