P3: align DRIFT timestamp validation with the ASTER CompiledBundle contract

[tuzuminami]

Problem

ASTER strictly validates RFC 3339 timestamps and calendar dates, while DRIFT accepts any Date.parse()-parseable value. DRIFT can therefore accept malformed producer metadata that ASTER itself would reject.

Acceptance Criteria

• Implement the ASTER-equivalent RFC 3339 and calendar-date validation in the DRIFT consumer.
• Import/maintain a shared invalid-timestamp corpus in consumer contract tests.
• Reject producer-invalid / consumer-accepted bundles even when their canonical hash is recomputed.
• Keep the compatibility fixture pinned to a public ASTER release artifact.