feat: add retry-safe weekly period lock contract

[Pigbibi]

Scope

Fresh clean-slate PR-0 from the latest origin/main; does not reuse PR #28 code or commits.

Pure pert.weekly.period_lock.v1 only:

• immutable PoliticalEventWeeklyPeriodLockV1 and SourceSnapshotArtifact values;
• exact UTC ISO Monday/Sunday period identity and original workflow/run evidence;
• producer full SHA, source snapshot id/digest/counts/provenance;
• canonical JSON serializer/parser with duplicate-key, unknown/missing, unsafe-int, path, tamper and period mismatch fail-closed behavior;
• exact expected-lock comparison; generated_at is intentionally absent and remains mutable in later producer builds.

No workflow, lock acquisition/upload/download, fetch, artifact, QAR, Pages, publisher, monthly or permission changes.

Validation

• focused period-lock/weekly contract/manifest tests: 57 passed
• full pytest: 87 passed
• python3 -m compileall -q src tests: passed
• git diff --check: passed
• ruff: unavailable locally

Next slice must separately implement original-run lock and immutable input-snapshot acquisition; missing evidence must fail closed rather than fall back to wall clock.

[github-actions]

🤖 Codex PR Review

✅ Merge allowed: No issues found

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Review by Codex PR Review bot • PR