ci: start release candidates beside e2e

[numnx]

Summary

• start the release-candidate desktop package matrix immediately after package smoke instead of waiting for E2E and orchestration to finish
• keep the E2E, orchestration, and release-candidate matrices individually bounded while allowing GitHub to schedule them across the available runner pool
• update workflow health tests and CI docs for the new release-candidate lane timing

Verification

• pnpm exec vitest run tests/backend/ci/workflow-health.test.ts tests/backend/docs/testing-docs-commands.test.ts
• pnpm run check:docs-web
• pnpm run lint
• git diff --check
• workflow YAML parsed with PyYAML

Risk / Rollback

• Risk: peak eligible downstream jobs can exceed six by one (ci-dag 2 + e2e 3 + release-candidate 2), so GitHub may queue one matrix cell under the current runner quota.
• Rollback: restore release-candidate to needs: [package-smoke, ci-dag, e2e] if runner contention is worse than the release-candidate long tail.