test(buzz-db): replica_fence unit test fails on Linux due to sub-microsecond Utc::now() precision #2369

Summary

replica_fence::tests::fence_starts_closed_and_opens_on_advance fails deterministically on Linux (glibc), on a clean main checkout with no local changes:

assertion `left == right` failed
  left: Some(2026-07-22T14:04:14.529519Z)
 right: Some(2026-07-22T14:04:14.529519155Z)

Cause

The fence stores/returns the verified-through timestamp at microsecond precision, but the test compares it against the original Utc::now() value. On Linux, Utc::now() carries nanosecond precision, so fence.verified_through() == Some(ts) fails whenever now() lands on a non-zero sub-microsecond component (i.e. almost always). On macOS now() is microsecond-precision, which is why this passes in local dev and (presumably) CI runners there.

Repro

# Linux, clean main (7e34bee)
cargo test -p buzz-db --lib fence_starts_closed
# fails 3/3 runs

Suggested fix

Either truncate the expectation in the test (ts.trunc_subsecs(6) / compare through the same storage round-trip), or store nanoseconds in the fence. Test-side truncation is the minimal change if microsecond storage is intentional (matches Postgres timestamptz precision).