cttp: stream outbound %progress per chunk via stream flag

[mybropro]

Summary

Implements per-chunk %progress delivery for outbound HTTP requests handled by cttp.

When the iris %request gift arrives with stream=%.y, cttp:

• fires a %start with empty body and complete=%.n on header receipt (so iris records the response-header)
• fires a %continue per h2o body callback with the chunk and complete=%.n
• fires a terminating %continue with empty body and complete=%.y on EOS

Default stream=%.n preserves the existing buffered path — a single %start with the full body and complete=%.y.

Wire-format change

The iris %request gift to the %i driver gains a trailing stream=? field. _cttp_ef_http_client now parses a 3-tuple [num req str] instead of [num req]. This is a coordinated change with urbit/urbit#7342.

VERE_ZUSE bumped 408 → 407 in both pkg/vere/mars.c and pkg/vere/pier.c to match the lull/iris bump.

Cannot land until

The companion arvo PR — urbit/urbit#7342 — is in. This vere PR can be reviewed in parallel but the kelvin assertion will refuse to boot against an unupgraded arvo.

Motivating use case

%llmproxy, an Urbit desk that proxies LLM inference to a local Ollama backend. Ollama streams responses as newline-delimited JSON; without %progress support the node agent must buffer the full response before forwarding anything over Ames.

Test plan

• Buffered regression — *outbound-config:iris (default stream=%.n) → single %finished with full body, no %progress
• Streaming — outbound-config:iris(stream %.y) → %progress per h2o body callback, then %finished with empty body
• Verified end-to-end on a fakezod with a local Python chunked HTTP server emitting random hex chunks of varied sizes

Follow-ons

• %cancel support for in-flight streaming requests (still unimplemented)
• Non-durable %progress injection (avoid event-log writes on a hot streaming path)

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