ci: add headless smoke + screenshot test for the bar

Launch the real Waybar binary inside a headless, software-rendered sway
compositor and verify it actually runs and renders — something the unit
tests and build jobs never do.

- test/smoke/run.sh: boots sway (WLR_BACKENDS=headless, pixman), starts
  waybar, asserts it stays alive with no fatal log, optionally grabs a
  screenshot with grim
- level 1: real modules (clock/cpu/memory/disk) load without crashing
- level 2: deterministic config is screenshotted and checked to be non-blank
- level 3: screenshot compared to test/smoke/reference.png (fuzz 8%,
  800px tolerance); screenshot + diff uploaded as artifacts
- .github/workflows/smoke.yml runs it on push/PR

The reference image must be blessed from a CI artifact (see test/smoke/README.md).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/smoke.yml

@@ -0,0 +1,80 @@
+name: smoke
+
+# Launch the real Waybar binary inside a headless sway compositor and verify it
+# starts, renders, and matches a reference screenshot. Complements the unit
+# tests and build jobs, which never actually run the bar.
+
+on: [push, pull_request]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  smoke:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends -y \
+            sway grim imagemagick fonts-dejavu-core \
+            meson ninja-build g++ pkg-config scdoc wayland-protocols \
+            gobject-introspection libdbusmenu-gtk3-dev libevdev-dev libfmt-dev \
+            libgirepository1.0-dev libgtk-3-dev libgtkmm-3.0-dev libinput-dev \
+            libjsoncpp-dev libmpdclient-dev libnl-3-dev libnl-genl-3-dev \
+            libpulse-dev libsigc++-2.0-dev libspdlog-dev libwayland-dev \
+            upower libxkbregistry-dev libxkbcommon-dev
+
+      - name: Build & install Waybar
+        run: |
+          meson setup build -Dman-pages=disabled
+          ninja -C build
+          sudo ninja -C build install
+
+      # --- Level 1: smoke with real modules ---
+      - name: Smoke — real modules load without crashing
+        run: ./test/smoke/run.sh test/smoke/modules-config.jsonc
+
+      # --- Level 1 + 2: launch deterministic config and screenshot ---
+      - name: Launch + screenshot (deterministic config)
+        run: ./test/smoke/run.sh test/smoke/config.jsonc test/smoke/style.css bar.png
+
+      # --- Level 2: the bar actually rendered something ---
+      - name: Check the bar is not blank
+        run: |
+          sd=$(convert bar.png -crop 1920x30+0+0 +repage -colorspace Gray \
+                 -format '%[fx:standard_deviation]' info:)
+          echo "bar strip stddev = $sd"
+          awk "BEGIN{ exit !($sd > 0.01) }" || {
+            echo "::error::the top bar strip looks blank — Waybar rendered nothing"
+            exit 1
+          }
+
+      # --- Level 3: golden comparison against the committed reference ---
+      - name: Compare against reference screenshot
+        run: |
+          if [ -f test/smoke/reference.png ]; then
+            ae=$(compare -metric AE -fuzz 8% test/smoke/reference.png bar.png diff.png 2>&1 || true)
+            ae=$(printf '%s' "$ae" | grep -oE '^[0-9]+' || echo 0)
+            echo "differing pixels (fuzz 8%): $ae"
+            if [ "$ae" -gt 800 ]; then
+              echo "::error::visual regression: $ae pixels differ from the reference (threshold 800). See the 'diff.png' artifact."
+              exit 1
+            fi
+            echo "✓ within tolerance"
+          else
+            echo "::warning::No reference image yet. Download the 'waybar-screenshot' artifact from this run and commit bar.png as test/smoke/reference.png to enable golden comparison."
+          fi
+
+      - name: Upload screenshots
+        uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: waybar-screenshot
+          path: |
+            bar.png
+            diff.png
+          if-no-files-found: warn