(#170) Wire demo/script + demo/frosting into build.yml

Run both demos as separate CI steps after the recipe build, so
regressions in either invocation surface in the same workflow as
the addin's own test suite. demo/sdk joins at v9.0.0 (Cake 6).

`if: success()` keeps the demos gated on the recipe build passing
first — they consume BuildArtifacts/temp/_PublishedLibraries/ which
only exists after DotNetCore-Pack runs.

Author: gep13

.github/workflows/build.yml

@@ -83,6 +83,24 @@ jobs:
           verbosity: Normal
           cake-version: tool-manifest
 
+      # The demo/ folders exercise the just-built addin DLL under
+      # Cake-major-matching cake.tool / Cake.Frosting versions. They run
+      # in their OWN tool/package context (independent of recipe.cake's
+      # cake.tool, which is constrained by Cake.Recipe's runtime). Both
+      # consume the addin from BuildArtifacts/temp/_PublishedLibraries/
+      # populated by DotNetCore-Pack. demo/sdk joins at the Cake 6
+      # catch-up release.
+
+      - name: Run demo/script
+        if: success()
+        shell: pwsh
+        run: ./demo/script/build.ps1
+
+      - name: Run demo/frosting
+        if: success()
+        shell: pwsh
+        run: ./demo/frosting/build.ps1
+
       - name: Upload Issues-Report
         uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5
         with: