ci: gate the Fable compile targets

.github/workflows/build-and-test.yml

@@ -30,3 +30,34 @@ jobs:
     # running anything.
     - name: Test
       run: dotnet run --project test --configuration Release --no-restore
+
+  # Compile-only gate for the Fable targets. The cross-target *suites* are not green yet
+  # (JS 82/84, Python 79/84, BEAM 14/84 — see CLAUDE.md), so they cannot gate anything, but
+  # compilation is binary and already passes on all three. This keeps a change to src/ from
+  # silently breaking a target between manual `just test-<target>` runs.
+  #
+  # Nothing is executed, only compiled, so no Node, uv, rebar3 or erl is needed.
+  # fail-fast is off so one broken target does not mask the state of the others.
+  fable-compile:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        lang: [beam, javascript, python]
+
+    steps:
+    - uses: actions/checkout@v5
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v5
+      with:
+        dotnet-version: 10.0.x
+        dotnet-quality: preview
+    # Pulls the `fable` tool pinned in .config/dotnet-tools.json (5.13.0).
+    - name: Setup paket
+      run: dotnet tool restore
+    - name: Install dependencies
+      run: dotnet restore
+    - name: Compile src to ${{ matrix.lang }}
+      run: dotnet fable src --lang ${{ matrix.lang }} --outDir build/gate-${{ matrix.lang }}