Xcode String Catalog (.xcstrings): insert keys textually to avoid whole-file re-dump churn

Loading Localizable.xcstrings with json.load and re-dumping it (even with indent=2, ensure_ascii=False) changes formatting across thousands of lines, because Xcode writes " : " separators and its own key order and escaping. Merges then conflict badly. To add keys from a script, find the sorted insertion point by scanning the top-level keys of "strings" and splice in a block formatted exactly like Xcode does (2-space indent, " : " separators). Then check that json.load still parses and that each locale has every key.