Timestamp validation accepts impossible values and values without an offset

[leoafarias]

Summary

_isIsoDateTime (lib/src/spec_rules/concept_rules.dart, unchanged on main) accepts any string that contains T and that DateTime.tryParse accepts. tryParse rolls out-of-range fields over instead of rejecting them, so impossible timestamps pass invalid-generated and invalid-verified:

Value	Accepted	Parsed as (UTC)
2026-13-40T17:46:35Z	yes	2027-02-09T17:46:35Z
2026-09-11T25:61:61Z	yes	2026-09-12T02:02:01Z
2026-02-30T10:00:00Z	yes	2026-03-02T10:00:00Z
2026-09-11T17:46 (no offset)	yes	the reader's local time zone

Date-only values are already strict: parseIsoDate round-trips its result and rejects non-canonical spellings. Timestamps have no equivalent check.

Proposal

• Add one strict timestamp parser, for example parseOkfTimestamp. It would accept the ISO 8601 extended format with an explicit offset, check each field's range, and round-trip the value.

• Use that parser everywhere a timestamp is read:

  • the invalid-generated / invalid-verified rules
  • OkfGeneration.atDateTime and OkfVerification.isUsable
  • staleness
  • graph metadata

  Today these paths separately accept String or DateTime through _scalarString, _parseDateTime and DateTime.tryParse. One parser simplifies them and makes them agree.

• Once Adopt OKF spec 62432a0: every timestamp is an ISO 8601 datetime with an explicit offset #56 lands, a missing offset should be reported too, because OKF now requires an explicit one.

The findings stay advisory, as they are today. The fix only makes them fire on invalid values.

[leoafarias]

Fixed in #59 (strict parser rejects 2026-02-30) and #60 (offset advisory + migrate). Shipped in 0.5.0.