mycelium defaults --tcp-listen-port and --quic-listen-port to 9651
when not provided. If anything else holds 9651 (a previous test
instance, a Docker container from the level-1 procedure, another
mycelium running on the host), the daemon comes up far enough to
serve the loopback API for a few seconds before tearing itself
down on the listen failure.
Pick three distinct ephemeral ports (api, tcp, quic) per spawn.
Trade-off: inbound peers need the actual port number, which we
already log; the user can pin via a future SidecarConfig field.
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