fix(ci): install .NET 10 via script to writable DOTNET_INSTALL_DIR

setup-dotnet fails on ARC runners (no permission to /usr/share/dotnet) and dotnet
is not pre-installed. Install via dotnet-install.sh into $HOME/.dotnet instead,
then add to PATH via GITHUB_PATH.

.github/workflows/ci-tests.yml

@@ -17,8 +17,14 @@ jobs:
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
 
-      # setup-dotnet omitted — self-hosted ARC runners already have .NET 10 installed
-      # setup-dotnet fails on these runners (cannot write to /usr/share/dotnet)
+      # Install .NET to a writable location — self-hosted ARC runners don't allow
+      # writes to /usr/share/dotnet, so we redirect via DOTNET_INSTALL_DIR.
+      - name: Install .NET SDK
+        run: |
+          export DOTNET_INSTALL_DIR="$HOME/.dotnet"
+          mkdir -p "$DOTNET_INSTALL_DIR"
+          curl -fsSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin --channel 10.0 --install-dir "$DOTNET_INSTALL_DIR"
+          echo "$DOTNET_INSTALL_DIR" >> "$GITHUB_PATH"
 
       - name: Run DotNet CLI Tests
         run: >