Stream JSON report I/O

[steve-the-edwards]

Large Gradle builds can exhaust their heap when encodeToString materializes aggregate report payloads alongside the collected model.

• Stream JSON and JavaScript report payloads directly to their output files.
• Stream intermediate and historical JSON reads instead of loading whole-file strings.
• Count serialized bytes without materializing strings when deciding stat chunk boundaries.
• Preserve the existing UTF-8 JSON and JavaScript output byte-for-byte.

Additional context

Aggregate report serialization keeps the full collected model live. Streaming file-backed JSON conversions removes the additional growable JSON string from peak heap usage. Browser-side string conversions remain where Kotlin/JS APIs require JavaScript strings.