test: TestRoutePatterns410 hidden-hop test is flaky across a second boundary (two time.Now seeds) #483

Problem

TestRoutePatterns410_HiddenHopWithTrafficLooksUnknown/observer_blacklist (added in #453) failed in CI with a one-second firstSeen split (19:49:05Z vs 19:49:06Z). The test seeds its fixture from two separate time.Now() calls, one near line 45 and one near line 722. When those fall in different seconds, the hidden and unknown responses differ only in that timestamp, and the "byte-identical" comparison fails. It was reported in the round-2 review of #478 (G6) and is unrelated to that PR.

Task

Make the test deterministic. Use one fixed reference time for every seed, or compare with the timestamp normalized only where the test's intent allows it. Do not weaken the hidden-vs-unknown indistinguishability check, which is the privacy property from #410. No retries.

Acceptance criteria

50 consecutive go test -run TestRoutePatterns410 -count=50 runs pass. A mutant that makes a hidden hop distinguishable still fails.