[codex] Add TCP JSONL stream framing for cross-process modules

[superposition]

Closes #92

Summary

• Add a versioned NetworkStreamFrame contract for TCP JSONL stream messages.
• Add async helpers for reading/writing frames and one-message localhost TCP send/accept proof.
• Export the frame in the public crate API and generated schema artifact.
• Document the boundary as the first cross-process module stream contract while keeping runtime defaults unchanged.

Validation

• cargo fmt --check
• cargo run --features mcp --bin leash-schema -- --check
• cargo test transport::tests --all-features
• cargo check --all-features
• cargo test --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• scripts/smoke-all.sh
• cargo test --no-default-features --features sim,mcp
• cargo package --locked --allow-dirty
• cargo package --locked