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/config.jsonc

@@ -0,0 +1,26 @@
+// Deterministic Waybar config for the CI smoke/screenshot test.
+// Uses only static custom modules (interval "once") so the rendered bar is
+// reproducible and can be compared against a reference image.
+{
+    "layer": "top",
+    "position": "top",
+    "height": 30,
+    "modules-left": ["custom/left"],
+    "modules-center": ["custom/center"],
+    "modules-right": ["custom/right"],
+    "custom/left": {
+        "format": "Waybar",
+        "exec": "echo",
+        "interval": "once"
+    },
+    "custom/center": {
+        "format": "CI smoke test",
+        "exec": "echo",
+        "interval": "once"
+    },
+    "custom/right": {
+        "format": "12:34",
+        "exec": "echo",
+        "interval": "once"
+    }
+}