Tune Tokio TCP socket buffers

[rgbkrk]

Summary

This creates a fresh, narrow PR inspired by the old #265 cloud-agent branch.

Changes:

• request 4 MiB TCP receive and send socket buffers for Tokio TCP connections
• apply the tuning on both outgoing connect() sockets and accepted listener sockets
• keep the tuning best-effort by logging failures at debug level and continuing connection setup
• leave non-Tokio runtime builds on the existing behavior through a no-op helper

Notes

This intentionally preserves socket semantics and public API shape. The only behavior change is the best-effort OS socket-buffer request on the Tokio TCP transport path.

Validation

rustfmt --edition 2021 --check src/transport/tcp.rs
cargo check --features tokio-runtime,tcp-transport
cargo check --no-default-features --features async-std-runtime,tcp-transport
cargo check --no-default-features --features async-dispatcher-runtime,tcp-transport,async-dispatcher-macros
git diff --check
cargo test --test connect --test large_message

[rgbkrk]

Closing this duplicate in favor of #281. The two PRs have the same patch-id against current master, and #281 carries the fuller context/body for the TCP buffer tuning work.