Add TCP JSONL stream framing for cross-process modules

[superposition]

Parent: #12

Goal

Add a small no-hardware network stream framing contract so external module processes can exchange Leash stream messages over localhost TCP without knowing whether the peer is in-process or out-of-process.

Scope

• Define a versioned TCP JSONL stream frame for StreamMessage payloads.
• Add standard-library/Tokio helpers to write and read newline-delimited frames over TCP.
• Prove loopback cross-process-style communication in tests without requiring hardware or external services.
• Document the contract as the first cross-process/network transport boundary while keeping memory and local-pubsub as the default runtime transports.

Acceptance

• cargo test covers a loopback TCP sender/receiver exchanging a stream message.
• The JSONL frame includes a stable schema/version marker and rejects unsupported versions.
• Existing runtime stream transport behavior remains unchanged by default.
• README/docs explain when to use the TCP JSONL boundary and what remains out of scope.

[superposition]

Merged to main via PR #93: #93

Merge commit: f8271eba642ec12c3ce36462e0b6f573e86975ed

Local proof:

• 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 (ok: true)
• cargo test --no-default-features --features sim,mcp
• cargo package --locked --allow-dirty
• cargo package --locked

PR CI proof: all gh pr checks 93 jobs passed, including both rust jobs and every feature-matrix job.