Flaky unit test: UdpHeartbeatTest reads the sent count before it is updated #247

UdpHeartbeatTest.sendsOneOneEverySecondUntilClosed fails now and then, locally and possibly in CI; it failed 2 of 4 runs of the unit tests in Docker on 2026-10-20 (the other 358 tests pass):

java.lang.AssertionError
    at com.openipc.decoder.UdpHeartbeatTest.sendsOneOneEverySecondUntilClosed(UdpHeartbeatTest.java:102)

Line 102 is assertTrue(h.sent() >= 4);, right after the test has received the 4th packet. The heartbeat counts a packet after send() returns, while the test's receive() can return as soon as the datagram arrives, so the count may still be 3 when it is read. The test races the counter; the heartbeat itself works.

Fix: wait briefly for the count (or assert >= 3 and check the 4th after close()), so the test does not depend on thread timing. Test-only change, no new version (no-release).