First-pass JSON formatter test: try nested and escaped data

Before reformatting a large JSON document, use a harmless sample with nested objects, arrays, Unicode escapes, and nulls. Format it and compare parsed values and types with the source. Add a duplicate-key example to understand the tool's behavior, since parsers may discard one value. If the sample changes data, stop and adjust the workflow. Even if it passes, compare the final document separately, especially when exact bytes are used for signatures.