Summary

• optimize Serialize.Writer.schedule_iovecs to advance an iovec cursor incrementally across DATA chunks
• avoid repeatedly calling IOVec.shiftv from the original iovec list for each chunk
• preserve frame splitting and END_STREAM semantics

Tests

• add regression test fragmented request body chunking in lib_test/test_h2_client.ml
  • writes many fragmented body chunks
  • verifies emitted DATA payloads concatenate back to the original body
  • verifies body is split across multiple DATA frames

Validation

• dune runtest --no-buffer