1. Both WinRT and std::filesystem use system-native UTF-16 paths on Windows. Defaulting to UTF16 lets us avoid conversion until needed, and fixes some instances of round-tripping between formats.
2. We rely on `path.u8string()` to return a Unicode representation of a path, but that API in C++ 20 returns `std::u8string` which is incompatible with every other existing utf-8 API. `path.string()` may convert to system codepage instead.
3. This makes coroutine usage more efficient when using `IAsyncOperation<winrt::hstring>`, the pattern to replace `std::future<std::string>` usage mechanically.
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