dotfiles: fix two architecture bugs a real ARM64 run exposed

Running setup-windows.bat end to end on an ARM64 box turned up two
packages that quietly did the wrong thing, and one bug in the audit that
was hiding a third.

WinMerge installed as x64 on ARM64. Upstream publishes the ARM64 build as
a MACHINE-scope installer and the x64 build as a PER-USER one, so an
unelevated winget lets its scope preference beat its architecture
preference and silently picks the emulated x64. Ask for --architecture
arm64 first and fall back to the default, so an administrator run gets the
native build and a standard-user run still gets a working diff tool.

LLVM does not fail when it cannot write to Program Files - it relocates.
The NSIS installer falls back to a per-user directory (observed at
%USERPROFILE%\Documents\LLVM) and winget still reports "Successfully
installed", so the compiler is genuinely present, genuinely native, and
findable by nothing. Its silent install also does not take the add-to-PATH
option. New LlvmPath step looks in that fallback as well as the usual
roots, reports the architecture, notes when the location means the install
was not machine-wide, and appends the directory to the user PATH.
Appended rather than prepended: this is a second compiler kept alongside
MSVC and should not quietly win a clang-cl meant for Visual Studio's copy.

The audit resolved labelled entries PATH-first, so with upstream LLVM on
the PATH the "clang-cl.exe (VS)" row answered with the upstream compiler
and reported Visual Studio's as present when it was absent. A labelled
entry names one specific copy and is now resolved only from its explicit
candidates; unlabelled entries still resolve PATH-first, because for those
the question is which binary a build would actually invoke.

Also record what a standard user cannot provision. The elevated half needs
an account that can elevate, and several packages in the non-elevated
section are per-user in name only: the .NET SDK (exit 5), CMake and
Android GPU Inspector (MSI 1603) and OpenCppCoverage (self-elevating) all
fail without an administrator.

Verified on ARM64: cmake, ninja, git, python, ssh, xperf, MSBuild and
cl.exe all native; both compilers build a native ARM64 binary through the
Ninja generator - MSVC 1950 and clang 22.1.8, each reporting target ARM64.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ak6VzfGyaocrykX1LhNMwt