Fix storefront routing and Ubisoft library visibility

[aaf2tbz]

Summary

Fix BeamNG.drive and The Witcher 3 executable selection across storefronts, and ensure an installed Ubisoft copy isn't hidden by an uninstalled Steam entry.

Changes

• Select BeamNG.drive's 64-bit executable for Steam and stage D3DMetal DLLs beside it.
• Resolve witcher3.exe for Steam, Epic, and GOG instead of launching REDlauncher.
• Prefer installed Ubisoft library entries when Steam only lists the same title as uninstalled.

PR Readiness (MANDATORY)

• Real-game launch test completed for game/runtime/graphics/compatibility changes (game + method below; otherwise mark N/A)
• No hardcoded paths, secrets, or absolute /Users/... paths introduced
• Config/rules TOML validated if configs/mtsp-rules.toml or DLL maps changed
• Version metadata (CMakeLists.txt, app/src-c/Makefile, package.json, package-lock.json) in sync if version bumped
• Bottle/runtime migration and launch behavior preserved (rollback plan noted if changed)
• Docs / compatibility matrix updated for user-facing changes
• Regression test added for each bug fix

Local toolchain

• C backend builds and tests: make -C app/src-c test (runtime-ready simulation skipped because bundle archives are absent)
• clang-format --dry-run --Werror passes on changed C files
• TypeScript compiles: cd app && npx tsc --noEmit
• Biome and Prettier pass for renderer files
• Shell scripts lint: tools/ci/shellcheck.sh
• tools/ci/validate-rules-toml.py passes
• git diff --check passes
• No new files added to the repo root

Test notes

• User-tested BeamNG.drive through Steam and Far Cry 5 through Ubisoft Connect; both launched, and Far Cry 5 appears in the library.
• Runtime route regression tests cover BeamNG's 64-bit executable selection and Witcher 3 resolution; the GOG smoke test verifies the --override-exe argument.
• Renderer build and local C backend tests pass. The C backend test run skipped only the runtime-ready simulation because required bundle archives are not present locally.

Risk

Low. Changes affect per-game executable selection and library merging. Rollback is to revert this PR.