alongsideMixinOverlay

Run main with the mixin overlay building next to it, and report whether that overlay published.

Its own thread because it is jar I/O and outlasts the shard pool, so it starts first and is joined after — it touches no Mixin state, MixinProbe having resolved all of that to plain data before the plan existed. A failure costs only the declarations: the answer is false and nothing stale is prepended, while the shards still stamp — snippets just have to cast to reach what Mixin merged, until the next rebuild retries.