Provide a core streaming response helper #110

Description

Provide reusable core primitives for building streaming responses without requiring a known content length. HTTP wire framing belongs to the adapter.

Body::stream and fallible Body::from_stream construct portable streaming bodies. response_with_body/IntoResponse accept them without adding a buffered content length, and the streaming guide documents chunked transfer and adapter limitations.

Steps to reproduce

1. Inspect the core body constructors and response helper.
2. Inspect the streaming guide section describing unknown-length responses and chunked transfer.

Expected behavior

A handler can return a streaming body through a standard core response helper; capable adapters choose the appropriate transfer framing.

Impact

The requested helper is already available; all-adapter progressive delivery remains tracked separately.

Proposed fix

Close as completed using the existing body/response API. Do not hardcode an HTTP/1.1 Transfer-Encoding header into the portable core.

Done when

• Core exposes infallible and fallible streaming body constructors.
• Core response construction accepts a stream without buffering it or inventing its length.
• Documentation explains unknown-length/chunked delivery and the runtime-specific buffering limits.

Related

• Parent: Complete streaming and compression across adapters #107.
• Adapter delivery work: Stream Fastly standard responses progressively #109 and Axum: preserve incremental streaming with bounded backpressure #394.
• Portable streaming body constructors
• Response helper and buffered-length handling
• Chunked-transfer documentation

The requested implementation is present on the audited main revision. This audit inspected source and existing coverage; it did not rerun the tests.