Make the integration test deterministic

[stepchowfun]

toast validate_release was flaky because the integration test waited for Docuum to finish processing events by sampling the process state in /proc, which can't distinguish "idle, waiting for the next event" from "blocked on I/O mid-vacuum", and which also raced against Docker's asynchronous event delivery.

The test now synchronizes on Docuum's own logs instead:

• Docuum runs with LOG_LEVEL=trace, so it logs every incoming event before processing it.
• After each step, the test emits a sentinel event by creating and removing a uniquely named Docker network, then waits for Docuum to log its arrival. Docker delivers events to a subscriber in order and Docuum processes them strictly sequentially, so receipt of the sentinel proves all earlier events were fully processed. Network events are irrelevant to Docuum, so the sentinels don't perturb the state under test.
• Containers are removed explicitly rather than with --rm, whose asynchronous removal could emit events after the sentinel.
• At startup, the test probes with network events until Docuum reports receiving one, closing the race between spawning docker system events and its subscription being established.

The two trace/info log lines the test greps for are now tagref-tagged ([tag:incoming_event], [tag:listening_for_docker_events]), so changing them without updating the test fails toast lint. The src/run.rs changes are comments only.

Verified by running toast validate_release locally end to end.

Status: Ready

Fixes: N/A

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