Disable mscompatdb for M12 launches

[aaf2tbz]

Summary

• explicitly disable the legacy mscompatdb Wine sidecar for M12 via WINEDLLOVERRIDES
• keep the existing M12 native/builtin D3D12/DXMT routing unchanged
• add regression coverage for the engine definition and Steam handoff env

Root Cause

M12 includes lib/wine/x86_64-unix in its runtime route, which leaves Wine free to load the old v1 mscompatdb.so TOML hook. That legacy sidecar is not part of the isolated M12 DXMT route and can fail launch before the game reaches the D3D12 path.

Validation

• cargo test mtsp::engine::tests::m12_is_stronger_than_other_dxmt_d3d_paths
• cargo test mtsp::launcher::tests::steam_pipeline_env_includes_route_overrides_and_cache_keys
• cargo test mtsp::engine
• cargo test mtsp::launcher

[aaf2tbz]

Superseded by #207, which restores PR #204 onto current main and includes this M12 mscompatdb runtime override in the correct PR context.