Add runnable TCP JSONL stream hub

[superposition]

Closes #96

Summary

• add leash serve stream-hub for the TCP JSONL stream hub
• add the sim-stream-hub built-in stack on 127.0.0.1:9970
• add smoke coverage proving valid frames are accepted and invalid peers do not kill the listener
• document the runnable hub in the README, network transport example, and smoke script catalog

Validation

• cargo fmt --check
• bash -n scripts/smoke-all.sh scripts/smoke-stream-hub.sh
• bash scripts/smoke-stream-hub.sh
• cargo run --features mcp --bin leash-schema -- --check
• cargo check --all-features
• cargo test --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --no-default-features --features sim,mcp
• scripts/smoke-all.sh
• cargo package --locked --allow-dirty
• git diff --check
• cargo package --locked