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>

test/smoke/style.css

@@ -0,0 +1,27 @@
+/* Deterministic stylesheet for the CI smoke/screenshot test.
+ * A pinned font (DejaVu Sans, installed in CI) and fixed colors keep the
+ * rendered bar reproducible across runs for the golden comparison. */
+* {
+    font-family: "DejaVu Sans";
+    font-size: 14px;
+    border: none;
+    border-radius: 0;
+    min-height: 0;
+}
+
+window#waybar {
+    background-color: #1e1e2e;
+    color: #cdd6f4;
+}
+
+#custom-left,
+#custom-center,
+#custom-right {
+    padding: 0 16px;
+    color: #cdd6f4;
+}
+
+#custom-left {
+    font-weight: bold;
+    color: #89b4fa;
+}