corrective/intend: known-answer tests for the custom `timestamp.rs`

[hyperpolymath]

Type: corrective · Priority: intend

The hermetic SHA-256 (src/hash.rs) already pins NIST KATs, but the dependency-free RFC-3339 src/timestamp.rs (Hinnant civil-date algorithm) has no equivalent. Add round-trip + leap-year/boundary vectors so a calendar bug can't silently corrupt the event log.