Port cached trigger and nearest location policy

RE-1035. Focused validation: cargo xtask test-group rust-location-nearest. Machine report.

Rust matches16 original location-selection cases with signed cache expiry, supplied query order, trigger precedence, PVS filtering and later-wins distance ties. Typed indices and finite math reject unsupported domains. Cache values remain unchanged; tests show live cache bypasses unused invalid geometry and trigger selection bypasses the fallback list. Engine query/PVS and identity remain supplied inputs.