Lesson 4.2
The Response writer - serialize once at send time
A Response object that collects a status, headers, and a body, then serializes them to framed wire bytes exactly once, with content-length measured in octets and a one-way headersSent flag.
What you will build
- Explain why content-length can be computed correctly only at send time, and why it counts octets rather than characters.
- Distinguish chainable configurators from terminal methods by their return type and their content-type behavior.
- Build a Response whose single private _send is the one place framing happens, guarded by headersSent.