Guest page: the guest's composer and its own messages (carries #26819) - #30518

One shadow release compile runs at a time and one waits; another dispatch now would evict the waiting one. The scheduled pick compiles the newest feat-cmux-next head when the slot frees, and a PR's fallback compile has its own group.
Source SHA: 2f3724b3bb3ff9701b74fb5b744480c6597a25a6