feat(compliance): add validation result ids

[sangilish]

Summary

• add an optional storyboard validation id field for stable per-assertion references
• require runners to echo authored validation ids as validation_result.id
• bump the runner output contract from 2.5.0 to 2.6.0 and add a minor changeset

This is the spec-only slice for #5337. Addie pass-through and adcp-client runner emission remain separate follow-ups.

Validation

• npx prettier --check .changeset/validation-result-ids.md
• rg -n 'version: "2\\.6\\.0"|validation_result\\.id|id: string \\(optional stable|echo this value' static/compliance/source/universal/storyboard-schema.yaml static/compliance/source/universal/runner-output-contract.yaml .changeset/validation-result-ids.md\n\nNote: I did not reformat the touched contract YAML files because runner-output-contract.yaml already fails full-file Prettier/YAML parsing on main; this PR keeps the diff to the contract change only.

[aao-release-bot]

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