Platform expressions cannot express "when the host is Windows" for host dependencies

[omartijn]

Ports whose Windows build needs a host-only helper (e.g. vcpkg-msbuild, which has "supports": "native & windows") have no correct platform expression for that dependency:

• "platform": "windows & !mingw" is evaluated against the target triplet, so cross-compiling e.g. libsodium:x64-windows from a Linux host fails during planning with "vcpkg-msbuild is only supported on 'native & windows', which does not match x64-linux" — even when the port would take a non-msbuild code path for that build.
• "platform": "windows & !mingw & native" fixes cross resolution, but native compares triplet names, so any Windows triplet named differently from the host triplet regresses: libsodium:x64-windows-static on an x64-windows host is not native, the dependency is dropped, and the msbuild path fails. See the review discussion in [libsodium] support non-MSVC compilers on Windows via the make-based … vcpkg#52803, where exactly this trade-off came up.

What seems to be missing is a way to express "this dependency is needed when the host can run it / is Windows". Possible directions:

• evaluate the platform expression of "host": true dependencies against the host triplet instead of the target triplet (arguably the intuitive semantics, but a behavior change);
• or add a host-context predicate to platform expressions (e.g. host_windows, or a general $host(...) context).

The workaround today is --allow-unsupported, which turns a targeted constraint into a blunt instrument for the whole install plan.