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/modules-config.jsonc

@@ -0,0 +1,19 @@
+// Waybar config for the CI smoke test: exercises real modules that work in a
+// headless environment (no hardware/daemon dependency). Its content is dynamic
+// (time, cpu, ...) so it is NOT screenshot-compared — it only checks that these
+// modules load and render without crashing the bar.
+{
+    "layer": "top",
+    "position": "top",
+    "height": 30,
+    "modules-center": ["clock", "cpu", "memory", "disk", "custom/hello"],
+    "clock": {},
+    "cpu": {},
+    "memory": {},
+    "disk": {},
+    "custom/hello": {
+        "format": "ok",
+        "exec": "echo",
+        "interval": "once"
+    }
+}