WinRT.Host misidentifies standard casing and probes outside its directory

Description

WinRT.Host.dll is the standard non-NativeAOT implementation name emitted by Microsoft.Windows.CsWinRT.Authoring.targets, but WinRT.Host.cpp recognizes its generic-host convention only when the host filename is exactly lowercase winrt.host.dll.

if (host_file.wstring() != L"winrt.host.dll")

On Windows, a normal WinRT.Host.dll is therefore treated as a renamed host. The renamed-host probing then repeatedly calls rfind('.') on the complete module path:

std::size_t count = target_path.rfind('.');
target_path.resize(count);

After exhausting the filename suffixes, this can consume a dot in a parent directory and probe outside the host directory.

Minimal reproduction

1. Put the generic host and WinRT.Host.runtimeconfig.json in a dotted directory, for example C:\build\Sample.Packaging\AppX\WinRT.Host.dll.
2. Put the actual authored assembly next to the host, for example C:\build\Sample.Packaging\AppX\Contoso.Component.dll, and activate Contoso.Component.Widget.
3. Place an unrelated C:\build\Sample.dll adjacent to Sample.Packaging.
4. Activate the runtime class.

Actual probe sequence includes:

C:\build\Sample.Packaging\AppX\WinRT.Host.Server.dll
C:\build\Sample.Packaging\AppX\WinRT.Server.dll
C:\build\Sample.Packaging\AppX\WinRT.dll
C:\build\Sample.Server.dll
C:\build\Sample.dll

The last candidate is outside the host directory. If it exists, the host loads it and class lookup can fail with CLASS_E_CLASSNOTAVAILABLE; otherwise class-name convention probing never reaches the expected Contoso.Component.dll.

Expected behavior

A generic host installed as the normal CsWinRT-authored WinRT.Host.dll should use class-name convention probing and resolve Contoso.Component.Widget to Contoso.Component.dll beside the host. Renamed-host probing must never traverse above the host directory.

Proposed fix

1. Compare the generic-host filename case-insensitively.
2. For renamed hosts, peel dot-separated suffixes from host_module.filename() only, then combine each candidate filename with the original parent path.

Affected versions

The source file is identical in CsWinRT 2.3.1 and 3.0.0-preview.260319.2 (src/Authoring/WinRT.Host/WinRT.Host.cpp, blob 82566f5995536eb4d4432058db7ee2d57c2d5570), so the current 3.0 preview is also affected.

Start with src/Authoring/WinRT.Host/WinRT.Host.cpp and reproduce the issue using a dotted host directory and the filenames in the report. Check the generic-host filename comparison and the renamed-host probe candidates. Done means normal WinRT.Host.dll resolves the authored assembly beside it, while renamed-host probing does not leave the host directory.