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.
The wrapper adds no output when configuration succeeds and is not executed if the
entry file cannot be read. Mutare.Sandbox.Command.Output reads the marker alongside the
exception header.