Respect the import and export boundary

The JSON import and export tools do not expose public row-transformation hooks. Their versioned structure, exact-source matching, allowed statuses, sanitization, placeholder validation, and existing-value policies are fixed by the current tool.

Do not filter upload handling or call its private AJAX actions as an integration API. Produce a standard POM Translate export when portability is required.

After import, normal successful writes fire pom_translate_translation_changed, which can invalidate integration-owned caches. Missing, empty, existing, or invalid entries that are skipped do not represent successful changes.

For another interchange format, build an external converter into the documented version-1 JSON shape and validate the resulting file through the administration tool.