--export-settings already writes the merged config as JSON at the right
point in the CLI flow. Passing - writes the same document to stdout.
- ConfigBase::save_to_json gains a stream overload. The file overload
serializes through it before opening the file, so the format is
unchanged and a config that cannot be serialized leaves an existing
file untouched instead of truncating it.
- On stdout, invalid UTF-8 in string values is written as U+FFFD instead
of ending the process with an uncaught type_error; files keep the
strict behaviour.
- - is rejected up front when combined with an action or transform that
can write to stdout or does real work, so stdout carries only the
JSON.
- The unconditional "skip locked instance" stdout write during arrange
now goes to the log.
- Tests in tests/libslic3r/test_config.cpp.