Parallel execution

WOML

<parallel
  id="checks"
  name="Run checks"
  description="Check stock and risk concurrently."
  concurrency="2"
  on-error="wait-all"
>
  <step id="stockCheck"><script>return checkStock();</script></step>
  <step id="riskCheck"><script>return checkRisk();</script></step>
</parallel>

Attribute	Required	Meaning
`id`	Yes	Stable parallel-group identity; not a context output.
`name`, `description`	No	Display metadata.
`concurrency`	No	Maximum active children; defaults to child count and cannot exceed it.
`on-error`	No	`fail-fast` or `wait-all`; defaults to `fail-fast`.

Rules:

It contains one or more direct <step> children only.
Every child sees the same pre-fork context.
Siblings cannot reference each other's output.
Each child publishes its own context.steps.<id> value.
The group has no aggregate output.
fail-fast stops unstarted children and requests cancellation of active children; wait-all lets all children settle. Both fail the group if any child fails.
Add a downstream step to create one final aggregate result.

Use <parallel> for several independent single-step operations. Use <fork> for concurrent lanes containing multiple steps.