CI builds x64 only while releases also build ARM64 #22

Problem

.github/workflows/build.yml builds and publishes win-x64 only:

- name: Build
  run: dotnet build md2loop/md2loop.csproj -c Release -r win-x64

.github/workflows/release.yml builds a matrix of win-x64 and win-arm64.

Impact

ARM64 is never compiled on push/pull_request. An ARM64-specific break — a native dependency, a RID-specific package resolution failure, a trimming difference — is only discovered when a v* tag is pushed, i.e. during a release, when it is most expensive to fix.

Suggested fix

Add a RID matrix to build.yml mirroring the release workflow:

strategy:
  matrix:
    rid: [win-x64, win-arm64]

Two additional considerations:

• The CI publish output is currently discarded. Uploading it as an artifact makes PR builds inspectable.
• CI publishes without -p:IncludeNativeLibrariesForSelfExtract=true, while release.yml and installer/build.ps1 both pass it. CI should use identical publish flags so it validates what actually ships.