#2699 gatorwalk-factory: run verification in parallel inside a stage, and document the pattern

Part of the gatorwalk-factory rebuild of @swamp/software-factory (gatorwalk-factory/ in swamp-extensions). Read gatorwalk-factory/README.md, DESIGN.md and lifecycles/swamp-extensions.md first. Launch requirement.

A gatorwalk lifecycle is in one stage at a time, and parallel stages are deferred (swamp-club #2665). What launch must have is running parts of the flow in parallel, above all verification. Today lifecycles/swamp-extensions.yaml, our re-implementation of issue-lifecycle, runs verify-build and verify-reviews as two stages one after the other, so a reviews failure only shows once the build passes and verification takes the sum of both times.

The pattern: do the concurrency inside one stage. The stage's work is a swamp workflow whose jobs run concurrently. Verified in swamp core at c48ef142: jobs at the same dependency level run in parallel, capped by the workflow's concurrency and SWAMP_MAX_CONCURRENT_STEPS (execution_service.ts:2299-2371), and a step can run a nested workflow (type: workflow, step_task.ts:52).

Scope:

1. lifecycles/swamp-extensions.yaml verifies in one stage that runs verify-build and verify-reviews concurrently and judges them together, the way the repo's checklist does. Record both outcomes as evidence, bound to the change-summary commit.
2. Check what the attestation needs: scripts/build_attestation.ts takes the verify-build and verify-reviews run ids. Find out whether nested runs keep their own run records and ids, and choose the wrapper design accordingly (a wrapper workflow with two nested-workflow jobs, or a stage dispatching both runs), so the attestation still builds. Say what you found.
3. Document the pattern in DESIGN.md and in the skill's references, as the way to run anything in parallel within a lifecycle, with the one-stage-at-a-time rule and why.
4. Tests: the lifecycle tests and a real-engine test showing the stage waits for both and fails if either fails.

Nothing is published.