Summary

Adds opt-in streaming mode to iris (cttp) HTTP client. When a request includes
an x-urbit-stream header, the runtime sends response data incrementally as
%start / %continue / %cancel events instead of buffering the entire body
into a single %finished event.

Motivation

The buffered response model requires the entire HTTP response to complete before
Arvo sees any data. This doesn't work for Server-Sent Events (SSE) streams,
which can stay open indefinitely.

How it works

1. Hoon-side code adds an x-urbit-stream: true header to the %request card
2. The runtime detects and strips this header before sending the request
3. Instead of accumulating body chunks, each chunk is dispatched immediately:
   • %start — headers arrived (with status code and response headers)
   • %continue — body chunk arrived (with optional data and fin flag)
   • %cancel — connection error (replaces the fake 504 in buffered mode)
4. Existing buffered behavior is completely unchanged for requests without the header