Stream Fastly standard responses progressively #109

Description

Make the standard Fastly dispatch path deliver response chunks progressively instead of constructing a complete host-side body before sending.

Main uses Fastly SDK 0.13, so the original SDK 0.11 migration prerequisite is obsolete. The remaining behavior gap is real: from_core_response drains the entire stream before returning. Custom lifecycle callbacks already permit application-managed streaming.

Steps to reproduce

1. Inspect Fastly SDK version and the standard response converter.
2. Compare standard dispatch buffering with the documented custom streaming callback.

Expected behavior

Standard Fastly streaming responses can commit headers and deliver chunks before end-of-stream, with clear error and completion behavior.

Impact

Long-lived streams and first-byte latency require custom sender code today.

Proposed fix

Complete the adapter-owned progressive egress path in #275, retain compatibility guidance for custom callbacks, and verify native delivery behavior before acceptance.

Done when

• A delayed/multi-chunk response emits its first chunk before the producer completes.
• Backpressure, stream error, completion and cancellation behavior are covered at the native Fastly boundary.
• Standard dispatch documentation no longer describes full buffering as the default streaming behavior.

Related

• Parent: Complete streaming and compression across adapters #107.
• Portable HTTP/lifecycle tracking: Implement portable outbound HTTP and request lifecycle contracts #436.
• Fastly standard response conversion
• Documented streaming boundaries
• Current Fastly SDK version
• Implementation PR feat: implement portable outbound HTTP and request lifecycle contracts #275; keep this issue open until its complete scope is accepted and merged.