Covers the Vercel wire translation: text deltas, streamed tool-call starts and argument fragments (including parallel routing by position), complete calls, results (string, structured, and null-filled), finish frames with renamed reasons, silently dropped chunk kinds, upstream errors reaching the reader, and response headers.

## Before writing SPEC.md files

Read https://raw.githubusercontent.com/brillout/sdd/refs/heads/main/sdd.md
