fix(compliance): validate top-level retry_after on 3.0.x

[bokelley]

Summary

• restore the rate-limit trip runner source contract on the 3.0.x line
• validate canonical top-level error.retry_after instead of the nonexistent nested path
• correct the same path in the universal idempotency storyboard
• remove integer-only assertions that the published numeric schema does not require

Why

The patch-line compliance source could mis-grade conformant sellers by looking under error.details.retry_after. The universal storyboard also referenced the wrong path, while its runner source contract was missing from this branch.

Validation

• storyboard test kits: 12 passed
• compliance build and included lints
• changeset scope/status checks
• git diff --check

No released dist artifact was modified and the number-versus-integer schema decision remains out of scope in #5953.

3.0.x portion of #5948. Merge the main-line fix #5954 first; the subsequent forward merge may require retaining main's broader corrected files.

[aao-secretariat]

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[aao-secretariat]

Argus is not auto-reviewing this PR because it modifies protected paths that require human review (.agents/playbook.md, .github/workflows/build-check.yml). A human reviewer should review and merge this PR; Argus will resume on subsequent PRs once these changes land on main.

[aao-secretariat]

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[aao-secretariat]

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[bokelley]

CI follow-up: the first failure came from the legacy 3.0.x freshness gate rebuilding already-published dist/compliance/3.0.21. I reverted that generated artifact change and backported main's immutable-release guard instead, removing the obsolete gate. This preserves released artifacts and lets the changeset flow create 3.0.22. Local immutable-artifact tests and the branch-vs-base guard both pass.