Keep Replacing a Read-Only Config and Include What the Helpers Need Everywhere

The config was always replaced by a rename, never written in place, so
the write helper's refusal of a read-only target, right for the presets
it mirrors, stopped a read-only OrcaSlicer.conf from ever being saved
again. The helper takes an explicit opt-in to replace such a file, and
the two config save variants share one writer that uses it.

The identity helper's stat headers were included only under the Linux
guard, which the Linux-only build never noticed; they are included on
every platform now. A temporary the longer name pushes past the path
limit is written in place like one refused for permissions, the rename
retry that waits for a Windows reader runs on Windows only, a failing
stat in the sweep skips that entry rather than ending the sweep, and the
rename fallback no longer stats the target it is about to remove.

src/libslic3r/AppConfig.hpp

@@ -451,7 +451,10 @@ private:
 
 	// Preset for each machine
 	MachineSettingMap											m_printer_settings;
-	// Has any value been modified since the config.ini has been last saved or loaded?
+	// Writes the assembled config text, and on Windows its checksum and a backup copy. `checksum_source`
+	// is the text load() will verify, which for the JSON config ends before the trailing newline.
+	bool												write_config_file(const std::string &path, std::string body, const std::string &checksum_source);
+
 	bool														m_dirty;
 	// Original version found in the ini file before it was overwritten
 	Semver                                                      m_orig_version;