[BUGFIX] Fix IR loading from emoji paths on Windows

[sdatkinson]

Summary

• preserve the UTF-8 IR filename received from iPlug2 instead of converting it back to a lossy narrow Windows path
• update AudioDSPTools so WAV files are opened through a native std::filesystem::path
• add coverage for loading a valid WAV from an emoji-named directory

Tests

• cmake --build AudioDSPTools/build/issue-641 -j4 (Release)
• ctest --test-dir AudioDSPTools/build/issue-641 --output-on-failure
• direct C++17 build and execution of test_wav

Notes

• Uses AudioDSPTools v0.1.2, which contains the merged Fix loading WAV files from Unicode paths AudioDSPTools#25 fix.
• The AudioDSPTools Debug build still encounters existing unrelated sign-comparison warnings promoted to errors in Resample.h; the Release build succeeds.

Resolves #641