Rust leaderboard mapping sentinels and joint metric selector

RE-777. Reproduce with python3 scripts/analyze_rust_leaderboard.py; validate with cargo xtask test-group gameplay. See the machine report.

Added typed metric records, sentinel initialization,20-slot key mapping and strict joint selector over caller-ordered candidates. Four Rust unit tests cover full slot range, all-sentinel nonselection, strict ties/crossing metrics, negative inactive gates and explicit nonfinite rejection. Candidate enumeration, category orchestration, native publication and runtime state ownership remain open.

No original executable/full VM/native world collision or rendering runs. Supplied callback/native responses and fixture domains remain explicit; complete macOS/Linux Rust/Bevy/Solari gameplay remains open.