CLI: let --export-settings - write the merged config JSON to stdout (#15698)

`--export-settings` already writes the merged config as JSON at the
right point in the CLI flow. Passing `-` now writes that same document
to stdout, so scripts can inspect the effective config without a temp
file. This replaces #14605.

- `ConfigBase::save_to_json` gains a stream overload. The file overload
serializes through it before opening the file, so the output format is
unchanged, and a config that cannot be serialized (invalid UTF-8) now
leaves the 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.
- To keep stdout pure JSON, `-` is rejected up front (stderr message,
`CLI_INVALID_PARAMS`, shell status 254) when combined with an action or
transform that can write to stdout or does real work: `--info`,
`--help`, `--orient`, slicing and exporting. Options that do nothing
without a slice (`--uptodate`, `--min-save`, `--pipe`, ...) are still
accepted.
- The one unconditional stdout write on a success path, "skip locked
instance" during arrange, now goes to the log.
- Every other value, including the default `output.json`, behaves as
before.

Tests in `tests/libslic3r/test_config.cpp`: the stream output equals the
file output and keeps the tab-indented format; invalid UTF-8 throws on
the strict path and is replaced when asked; a failed save leaves the
previous file intact.

src/OrcaSlicer.cpp

@@ -1387,6 +1387,25 @@ int CLI::run(int argc, char **argv)
     if (downward_check_option)
         downward_check = downward_check_option->value;
 
+    // --export-settings - writes its JSON to stdout, so reject every action or transform that may write there
+    // too (--info, --help, --orient, slicing and exporting). The allowed ones do nothing when nothing is
+    // sliced or exported.
+    if (std::find(m_actions.begin(), m_actions.end(), "export_settings") != m_actions.end() && m_config.opt_string("export_settings") == "-") {
+        static const std::set<std::string> stdout_compatible = { "export_settings", "uptodate", "load_defaultfila", "min_save",
+                                                                 "mtcpp", "mstpp", "no_check", "normative_check", "pipe" };
+        for (const std::vector<std::string> *opt_keys : { &m_actions, &m_transforms }) {
+            for (const std::string &opt_key : *opt_keys) {
+                if (stdout_compatible.count(opt_key) == 0) {
+                    std::string flag = opt_key;
+                    std::replace(flag.begin(), flag.end(), '_', '-');
+                    boost::nowide::cerr << "--export-settings - cannot be combined with --" << flag << std::endl;
+                    record_exit_reson(outfile_dir, CLI_INVALID_PARAMS, 0, cli_errors[CLI_INVALID_PARAMS], sliced_info);
+                    flush_and_exit(CLI_INVALID_PARAMS);
+                }
+            }
+        }
+    }
+
     bool start_gui = m_actions.empty() && !downward_check;
     if (start_gui) {
         BOOST_LOG_TRIVIAL(info) << "no action, start gui directly" << std::endl;
@@ -5348,7 +5367,7 @@ int CLI::run(int argc, char **argv)
                                 //skip this object due to be locked in plate
                                 ap.itemid = locked_aps.size();
                                 locked_aps.emplace_back(ap);
-                                boost::nowide::cout <<__FUNCTION__ << boost::format(": skip locked instance, obj_id %1%, instance_id %2%") % oidx % inst_idx;
+                                BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << boost::format(": skip locked instance, obj_id %1%, instance_id %2%") % oidx % inst_idx;
                             }
                         }
                     }
@@ -5937,7 +5956,11 @@ int CLI::run(int argc, char **argv)
             //FIXME check for mixing the FFF / SLA parameters.
             // or better save fff_print_config vs. sla_print_config
             //m_print_config.save(m_config.opt_string("save"));
-            m_print_config.save_to_json(m_config.opt_string(opt_key), std::string("project_settings"), std::string("project"), std::string(SoftFever_VERSION));
+            const std::string &settings_file = m_config.opt_string(opt_key);
+            if (settings_file == "-")
+                m_print_config.save_to_json(boost::nowide::cout, "project_settings", "project", SoftFever_VERSION, /*replace_invalid_utf8=*/true);
+            else
+                m_print_config.save_to_json(settings_file, std::string("project_settings"), std::string("project"), std::string(SoftFever_VERSION));
         } else if (opt_key == "info") {
             // --info works on unrepaired model
             for (Model &model : m_models) {