feat: adaptive V2 reconstruction with split/grouped range strategy

[XciD]

Summary

• Split large ranges (avg >= 256KB, <= 200 ranges) into individual single-range HTTP requests for better CDN cache hit rates and parallelism
• Keep small/fragmented ranges grouped in multi-range requests to avoid per-request overhead (~15ms per connection)
• Includes [BENCH] instrumentation (eprintln) for V1 vs V2 comparison: reconstruction timing, S3 response details, multipart parsing

This is the "V2-adaptive" approach from the benchmark results:

  label                  r/xorb |      V1 V2-orig V2-adpt |
 ----------------------------------------------------------
  gpt2_548MB                1.0 |    1.8s    2.0s    1.9s |
  smollm17b_3.4GB           1.0 |    6.0s    5.7s    4.2s |
  qwen35b_gguf_69GB        40.9 |  101.7s  301.2s   72.1s |

V2-adaptive is on par with V1 for non-fragmented files and 29% faster on highly fragmented ones.