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.
Escaping errors, exits, and throws print a marker before being re-raised with
their original class, reason, and stacktrace. Successful evaluation preserves
the source's value and prints nothing. Mutare.Sandbox.Command.Output requires
the marker and an exception header, even when deep calls lose all project frames.