Problem

Windows validation on PR #1659 failed in two unrelated actor-test races:

• Akka.Hosting.TestKit recovered a test actor lost during host startup, but strict message serialization rejected its internal probe-retarget message.
• The deferred reminder expiry test used a 700 ms global backstop and 200 ms wall-clock expiry without proving all execution slots were occupied. On Windows, one blocker completed early and the reminder under test started instead of being queued.

Fix

• Bind the narrow Akka.Hosting.TestKit StableTestProbeRef.UpdateTarget control message and add a strict-serialization round-trip regression.
• Replace the reminder test's real-time orchestration with FakeTimeProvider, three observed gateway deliveries as the saturation barrier, same-sender mailbox ordering, and an explicit delivery result to release one slot.
• Keep production and CI timeout thresholds unchanged. Do not bind Akka Streams internal stage messages.

Validation

• Focused regressions: 2/2 pass
• Affected classes: 62/62 pass
• Focused stress: 20 consecutive runs, all pass
• Full Netclaw.Actors.Tests: 2,627/2,627 pass
• Slopwatch: 0 issues
• Copyright headers: pass
• Diff check: pass