stored-at-stamp vectors: leap-day and year-boundary offset rows

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/vectors/stored-at-stamp/README.md

@@ -7,5 +7,5 @@ these answers, and a text that does not parse is refused `message_invalid` (fiel
 
 A stamp is an RFC 3339 date-time: `YYYY-MM-DDThh:mm:ss`, an optional fraction of 1 to 9 digits, and `Z` or a numeric offset
 `+hh:mm` / `-hh:mm` (hours 00-23, minutes 00-59). Uppercase `T` and `Z`, ASCII digits, no surrounding whitespace, a real
-calendar date, seconds 00-59 (no leap second), years 0001-9999. The reference is `parse_stored_at` in
+calendar date, seconds 00-59 (no leap second). The local year must be 1-9999 AND the UTC instant (after the offset is applied) must lie in years 0001-9999. The reference is `parse_stored_at` in
 `packages/cultcache-py/src/cultcache_py/stores.py` (#172); this file was written from it and is a reviewed commit when it changes.