ci: install mcpp from its release tarball, not the interactive installer

`curl | bash` of the xlings installer wants /dev/tty and exits 1 on a GitHub
runner — both legs died there before building anything. A released mcpp is
self-contained (it resolves its registry from beside its own executable), so
the tarball plus MCPP_HOME is all that is needed, which is what mcpp-index's
own CI does.

Pinned to 2026.8.27.2, mcpp-index's floor: consumers reach these packages
through that index, so that is the version worth testing against.

The llvm leg now selects its toolchain the supported way
(`mcpp toolchain install/default`) rather than a flag that does not exist.

Author: speak-agent

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 # Build every member on BOTH toolchains, and check what a green compile would
-# not: that the generator is ours, that the sonames are canonical, and that the
-# two libraries do not overlap.
+# not: that the generator is ours, that the sonames are canonical, that the two
+# libraries do not overlap, and that every module produced an interface.
 name: ci
 
 on:
@@ -10,31 +10,55 @@ on:
   pull_request:
   workflow_dispatch:
 
+env:
+  # The floor mcpp-index is tested against — consumers reach these packages
+  # through that index, so this is the version that matters, not the newest.
+  MCPP_VERSION: "2026.8.27.2"
+
 jobs:
   build:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        # Both, because the module wrapper is where they differ most: GCC and
-        # Clang disagree about what a `using ::name;` in an export block may
-        # name, and a 300-entry export list is exactly where that shows up.
-        toolchain: [gcc, llvm]
+        # Both, because the module wrappers are where the two compilers differ
+        # most: a 300-entry `using ::name;` export block is exactly the shape
+        # that finds a disagreement about what may be named in an export.
+        toolchain: [default, llvm]
     name: build (${{ matrix.toolchain }})
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install xlings + mcpp
+      # The release tarball, not the interactive installer: `curl | bash` of
+      # the xlings installer wants /dev/tty and exits 1 on a runner. A released
+      # mcpp is self-contained — it resolves its registry from beside its own
+      # executable — so MCPP_HOME is the tarball root rather than ~/.mcpp.
+      - name: Download mcpp
+        env:
+          MCPP_ARCHIVE: mcpp-2026.8.27.2-linux-x86_64.tar.gz
+          MCPP_ROOT: mcpp-2026.8.27.2-linux-x86_64
+        run: |
+          curl -L -fsS -o "$MCPP_ARCHIVE" \
+            "https://github.com/mcpp-community/mcpp/releases/download/v${MCPP_VERSION}/${MCPP_ARCHIVE}"
+          tar -xzf "$MCPP_ARCHIVE"
+          root="$PWD/$MCPP_ROOT"
+          echo "MCPP=$root/bin/mcpp" >> "$GITHUB_ENV"
+          echo "MCPP_HOME=$root" >> "$GITHUB_ENV"
+
+      - name: Select toolchain
+        if: matrix.toolchain != 'default'
         run: |
-          curl -fsSL https://d2learn.org/xlings-install.sh | bash
-          echo "$HOME/.xlings/subos/default/bin" >> "$GITHUB_PATH"
-          "$HOME/.xlings/bin/xlings" install mcpp -y
+          "$MCPP" toolchain install ${{ matrix.toolchain }}
+          "$MCPP" toolchain default ${{ matrix.toolchain }}
+          "$MCPP" toolchain list
 
       - name: Build the workspace
-        run: mcpp build --workspace --toolchain ${{ matrix.toolchain }}
+        run: |
+          "$MCPP" --version
+          "$MCPP" build --workspace
 
       - name: The macro mappings actually run
-        run: mcpp test -p mcpp/util --toolchain ${{ matrix.toolchain }}
+        run: "$MCPP" test -p mcpp/util
 
       - name: wayland-scanner is ours, and is 1.26.0
         run: |
@@ -73,17 +97,18 @@ jobs:
           readelf --dyn-syms -W "$s" | grep -q ' wl_display_connect' && { echo "server leaked the client API"; exit 1; }
           echo "client and server APIs are disjoint"
 
-      - name: both modules produced a BMI
+      - name: every module produced an interface
         run: |
           for m in client server util; do
-            find "mcpp/$m/target" -name '*.gcm' -o -name '*.pcm' | head -1 | grep -q . \
+            find "mcpp/$m/target" \( -name '*.gcm' -o -name '*.pcm' \) | head -1 | grep -q . \
               || { echo "$m: no module interface was produced"; exit 1; }
           done
           echo "wayland.client, wayland.server and wayland.util all have interfaces"
 
   upstream:
     # The fork patches no upstream file, so meson must still build this tree.
     # If this breaks, something was modified that should not have been.
+    name: upstream meson still builds this tree
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4