Fix truncated concatenated zstd response bodies - #470

Responses using Content-Encoding: zstd can contain multiple concatenated frames. xh currently stops after the first one, silently truncating the response body. In #469 this cuts a JSON response at 65,536 bytes, leaving jq with incomplete input.

ruzstd::StreamingDecoder intentionally handles a single frame. This change keeps xh's lazy read behavior but drives a reusable FrameDecoder directly, advancing through standard and skippable frames without exposing an intermediate EOF. Incomplete following frames and truncated skippable payloads still surface as decode errors, and transport errors retain their existing context.

Closes #469.