Use ordering equivalence for map subscripting

[perfloop]

C++ map membership follows ordering equivalence, not operator==. When those comparisons group keys differently, translated subscripting can return another value or replace an existing entry. Both generated map implementations now use the ordering relation for the hit test; explicit equality and lazy default construction are unchanged.

tests/unit/map_key_identity.cpp exercises both mismatches and matching keys. It checks values, size and traversal, construction count, and saved iterator and value-pointer identity. It also checks that explicit key equality keeps its C++ result. The fixture was not present at the accepted base; it fails on the test-only revision and passes after the shim change.

This condition holds with this change and does not hold at 24fc5a4f9d6a1f0f9af4cc1226c52e6953a16180: For the native and default-refcount translated executions of tests/unit/map_key_identity.cpp, std::map::operator[] uses key-order equivalence for this strict-weak-order comparator, creating one mapped value per ordered identity with stable saved iterators and value pointers while preserving the explicit C++ operator== results..

The checked-in C++ fixture and generated Rust expectations ship with this change. The native-versus-translated assertion runner and pinned Cargo locks are comparison support and are not part of the pull request. The unit, UB, and multi-file tests pass with the repository's existing expected-failure cases.

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