perf: adopt Swift 6.4 parsing optimizations

[vnixx]

Summary

• require Swift tools 6.4 while keeping existing Apple deployment targets
• centralize yyjson C-boundary reads through raw buffer/span helpers
• avoid Data copies in byte stream chunk reads
• add move-only JSONDocument storage with borrowed DOM traversal APIs
• document the streaming and borrowed traversal paths

Validation

• swift test -c release
• /Users/qianshou/ReerJSONBenchmark: swift run -c release EncoderBench

Benchmark Notes

• decoder paired benchmark: no material regression; Random Data improved about 3.8% across the paired runs
• encoder final check after reverting unproven hot-path cleanup: no regression observed in encoder-only benchmark
• borrowed JSONDocument traversal remains the main DOM win, measured around 30%+ in the earlier benchmark pass