Fix CI: build a cross-platform solution filter on non-Windows runners

HtmlRenderer.WinUI/HtmlRenderer.Demo.WinUI depend on the Windows App SDK's
MSIX PRI-generation step, which shells out to native Windows x86 binaries
(makepri.exe/makeappx.exe) - unlike the WinForms/WPF adapters, which
cross-compile cleanly everywhere via EnableWindowsTargeting's reference-
assembly story, there is no non-Windows equivalent for those native tools
at all ("Exec format error" on Ubuntu/macOS).

Adds Source/HtmlRenderer.CrossPlatform.slnf, excluding just those two
projects, and has the workflow build the full .sln on Windows but the
filtered one everywhere else - every other project (including the WinUI
adapter's own test-relevant siblings) still builds and tests identically
on all three OSes.

Author: jhaygood86

.github/workflows/build.yml

@@ -45,11 +45,23 @@ jobs:
     - name: Verify SDK Installation
       run: dotnet --info
 
+    - name: Select build target
+      id: build-target
+      run: |
+        # HtmlRenderer.WinUI/HtmlRenderer.Demo.WinUI (Windows App SDK/Win2D, MSIX PRI generation via
+        # native makepri.exe/makeappx.exe) can only build on Windows - unlike the WinForms/WPF adapters,
+        # which cross-compile cleanly everywhere via EnableWindowsTargeting's reference-assembly story,
+        # there is no non-Windows equivalent for those native tools at all. Non-Windows runners build a
+        # solution filter excluding just those two projects; everything else still builds and tests
+        # identically on every OS.
+        $target = $IsWindows ? "Source/HtmlRenderer.sln" : "Source/HtmlRenderer.CrossPlatform.slnf"
+        echo "path=$target" >> $env:GITHUB_OUTPUT
+
     - name: Restore Dependencies
-      run: dotnet restore Source/HtmlRenderer.sln
+      run: dotnet restore ${{ steps.build-target.outputs.path }}
 
     - name: Build
-      run: dotnet build Source/HtmlRenderer.sln --configuration Release --no-restore
+      run: dotnet build ${{ steps.build-target.outputs.path }} --configuration Release --no-restore
 
     - name: Run Tests
       if: runner.os == 'Windows'

Source/HtmlRenderer.CrossPlatform.slnf

@@ -0,0 +1,17 @@
+{
+  "solution": {
+    "path": "HtmlRenderer.sln",
+    "projects": [
+      "Demo\\Common\\HtmlRenderer.Demo.Common.csproj",
+      "Demo\\WinForms\\HtmlRenderer.Demo.WinForms.csproj",
+      "Demo\\WPF\\HtmlRenderer.Demo.WPF.csproj",
+      "HtmlRenderer\\HtmlRenderer.csproj",
+      "HtmlRenderer.PdfSharp\\HtmlRenderer.PdfSharp.csproj",
+      "HtmlRenderer.WinForms\\HtmlRenderer.WinForms.csproj",
+      "HtmlRenderer.WPF\\HtmlRenderer.WPF.csproj",
+      "Test\\HtmlRenderer.IntegrationTest\\HtmlRenderer.IntegrationTest.csproj",
+      "Test\\HtmlRenderer.PdfSharp.Test\\HtmlRenderer.PdfSharp.Test.csproj",
+      "Test\\HtmlRenderer.Test\\HtmlRenderer.Test.csproj"
+    ]
+  }
+}