Rust scoreboard numeric projection with source-specific ping and class policy

RE-807. Reproduce with python3 scripts/analyze_rust_scoreboard_rows.py; focused validation: cargo xtask test-group rust-scoreboard-rows. Machine report.

The scoreboard projection shares common status, signed-time, packed and cache arithmetic with recipient_payload but explicitly maps its connection gate and retains raw class values. Source parity checks all attempted rows, including capacity-rejected candidates. Unit tests cover signed wrap, bot/follow-cache precedence and invalid IDs without mutation. Byte formatting and packet admission follow in the next stage.

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