perf: streamline flight payload encoding
Context:
- Extract shared flight payload encoding helpers and avoid repeated JSON wrapper construction for binary chunks.
- Use chunked base64 conversion fallback and keep output format identical for client hydration scripts.
Benchmark (micro scenario, 300 iterations, 30 warmup, file-swap A/B):
- Web continueStaticPrerender median: 5.881ms -> 0.674ms (+88.55% faster)
- Web continueDynamicHTMLResume median: 5.708ms -> 0.773ms (+86.46% faster)
- Web continueDynamicPrerender median: 0.255ms -> 0.269ms (-5.35%)
- Web continueDynamicHTMLResume (utf8 flight) median: 1.187ms -> 1.329ms (-11.93%)
