perf(h3-quinn): hand DATA payloads to quinn without copying

[jothomas-xai]

What happens

h3-quinn's SendStream::poll_ready writes each chunk of a WriteBuf with quinn::SendStream::poll_write(cx, data.chunk()). quinn turns that &[u8] into an owned buffer in ByteSlice::pop_chunk (self.data[..limit].to_owned()), so every byte of every DATA frame is copied once into quinn's send buffer before it is copied again into a packet. #279 removed the copy on the h3-quinn side; this one is the copy inside quinn.

quinn already has a path that keeps the caller's buffers: SendStream::write_chunks(&mut [Bytes]).

Change

• h3-quinn: poll_ready takes each chunk out of the WriteBuf with copy_to_bytes and passes it to write_chunks. write_chunks is documented as cancellation safe ("If this does not resolve, no bytes were written"), so creating and polling a fresh future on each poll_ready is the same pattern quinn's own poll_write uses. A partially written chunk stays in writing (now Option<(WriteBuf<B>, Bytes)>) until quinn accepts the rest. No quinn changes.
• h3: WriteBuf overrides Buf::copy_to_bytes to delegate to the frame payload once the header is consumed. For a Bytes payload that is a split_to, so the payload reaches quinn without a copy. The frame header (a few bytes) is still copied, and a non-Bytes payload is copied once by its own copy_to_bytes, here instead of inside quinn.

poll_send (SendStreamUnframed) is unchanged.

Testing

• New unit test write_buf_copy_to_bytes_shares_payload_after_header: copy_to_bytes across the header/payload boundary copies, and after the header it returns a slice that shares the payload's allocation.
• finish_after_cancelled_send_data_keeps_connection_open sends a 4 MiB body that exceeds the stream window, so it exercises partial writes and Pending through the new path.
• cargo test --features 'tracing,i-implement-a-third-party-backend-and-opt-into-breaking-changes', cargo fmt --all -- --check, cargo clippy, ci/example_test.sh, and cargo +1.74.1 check --locked -p h3-quinn / -p h3 (lockfile from CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS=fallback) all pass locally.