idempotent_flush now also checks that an earlier `now` is clamped (no age moves), that a flush never ages an observation inserted after its `now`, that a release survives a flush, and that the store accepts inserts afterwards. Durability itself needs a restart, which the std-free suite can't do; tines-store tests it through the trait. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>