Rust stateful scoreboard broadcast transmission contract

RE-810. Reproduce with python3 scripts/analyze_rust_scoreboard_stream.py; focused validation: cargo xtask test-group rust-scoreboard-stream. Machine report.

The broadcast API scans state2 clients in original order and runs packet preparation between recipients. Input shape validates before mutations, even when no recipients qualify; no-recipient broadcasts preserve the ring counter. Ten original-control-flow fixtures match every transmission byte and metadata plus final cache/counter state. Native send and exit-rule state changes remain outside this pure transmission contract.

Original executable/native runtime and complete Linux/macOS Rust/Bevy/Solari acceptance remain unverified.