Preserve evidence of failures while evaluating the sandbox's mix.exs files.
Selection precedes project evaluation, so required library code and project/0
can fail with a mutation active, before configuration or application startup.
Wrapping the whole project source covers both inline and externally required
project definitions, independently of the compiler-options hook. The selector
and watcher bootstrap stays outside this wrapper so its own failures cannot
produce project evidence.
The wrapper prints a marker for escaping errors, exits, and throws, then re-raises
them with their original class, reason, and stacktrace. On success, the wrapper
returns the source's value and adds no output. Mutare.Sandbox.Command.Output requires
the marker and an exception header, even when deep calls lose all project frames.