Preserve evidence of errors, exits, and throws during runtime configuration.
Mix evaluates runtime.exs beside the effective config_path. That path can
be computed by project code, so sandbox materialisation wraps regular files
named runtime.exs throughout the project, without evaluating mix.exs or
following symlinks. Dependencies and excluded build/VCS trees are left alone.
The wrapper prints a marker only when a failure escapes evaluation, then
re-raises it with its original class, reason, and stacktrace. This covers imported
configuration and deep library calls even when the stacktrace has lost every Config frame.
Successful configuration prints nothing; an unreadable entry file never enters
the wrapper. Mutare.Sandbox.Command.Output reads the marker alongside the
exception header.