A comment explaining why the truncation guard sits on the type recorded how
the gap came to be noticed rather than the constraint itself, which AGENTS.md
rules out. The technical point is the one worth keeping: sqlite preserves
nanoseconds, so a sqlite-only suite cannot observe the truncation.
The neighbouring back-compatibility comment gets the same treatment — "clients
written before this existed" leans on the reader knowing what "this" was, and
naming conditional requests outright says it plainly.