test(archive): stamp expiry fixtures with one fixed time

[loganj]

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Summary

• The Rust test that checks archive storage expiry (storage_partition_dedup_policy_paging_and_shrinking) failed intermittently in CI when a runner was slow. It did not show a product bug.
• The test now gives all its fixture events one fixed timestamp, so it passes or fails the same way at any runner speed. The archive code is unchanged.

Details

• Before: each fixture event took its timestamp from the wall clock when it was made, but the expiry check ran relative to the first event's time. With a 1-day retention window, prune(now + 86401) removes only rows stamped no more than 1 second after now. If the test's 205 inserts took more than about a second, the later rows survived. Then the assertion "expired observer rows are gone" (tests.rs:185) failed. This happened in CI run 37623102186, attempt 1, where the archive tests took about 2 s. It passed on rerun.
• After: the test uses one timestamp for every event. The expiry boundary does not depend on how long the inserts take.
• Reproduction: a 2.1 s delay inside the insert loop makes the old test fail with the same assertion at the same line. With this change, it passes with the delay too.
• Other archive and relay tests that use these fixtures do not compare wall-clock stamps with a fixed time, so this is the only test changed.