CLI: let --export-settings - write the merged config to stdout

--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.

src/libslic3r/Config.hpp

@@ -2825,6 +2825,9 @@ public:
 
     //BBS: add json support
     void save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const;
+    // Same document, written to a stream. Invalid UTF-8 in a string value throws nlohmann's type_error unless
+    // replace_invalid_utf8 is set, which writes U+FFFD instead (for callers such as stdout with no handler).
+    void save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8 = false) const;
 
     // Rebuild the in-memory "plugins" manifest (the "name;uuid;capability" references the plugin
     // dispatchers consume) from the plugin-backed options via the registered resolver. save_to_json()