Port assistance acquisition and ownership guard

RE-1005. Focused validation: cargo xtask test-group rust-assist-acquire. Machine report.

Rust ports the conditional assistance ownership lock across36 source cases. Typed target indices preserve stale-state behavior, raw flag bits and wrapped clocks. Event104 precedes the original source log; both remain plans. Unit tests cover locked switching, inactive stale-target replacement, event ordering and atomic invalid-target rejection. Caller admission and proximity selection remain separate.