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/run.sh

@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# Launch Waybar inside a headless sway compositor and verify it starts and
+# renders. Used by the `smoke` CI workflow.
+#
+# Usage: run.sh <config> [style] [screenshot.png]
+#   - always: assert waybar launches, stays alive, logs no fatal error
+#   - if <screenshot.png> given: capture the bar with grim
+#
+# Requires: sway, grim, waybar (on PATH). Software-rendered, no GPU needed.
+set -euo pipefail
+
+CONFIG="$1"
+STYLE="${2:-}"
+SHOT="${3:-}"
+
+export XDG_RUNTIME_DIR="$(mktemp -d)"
+chmod 700 "$XDG_RUNTIME_DIR"
+export WLR_BACKENDS=headless
+export WLR_RENDERER=pixman
+export WLR_LIBINPUT_NO_DEVICES=1
+export LIBGL_ALWAYS_SOFTWARE=1
+
+LOG="$(mktemp)"
+SWAYCFG="$(mktemp)"
+
+wb="waybar -c $CONFIG"
+[ -n "$STYLE" ] && wb="$wb -s $STYLE"
+
+cat > "$SWAYCFG" <<EOF
+output HEADLESS-1 resolution 1920x1080 background #000000 solid_color
+exec "$wb -l debug > $LOG 2>&1"
+EOF
+
+echo "::group::Starting sway (headless) + waybar"
+sway -c "$SWAYCFG" &
+SWAY_PID=$!
+
+cleanup() {
+    swaymsg -q exit 2>/dev/null || kill "$SWAY_PID" 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Wait for the Wayland socket to appear.
+sock=""
+for _ in $(seq 1 40); do
+    sock="$(find "$XDG_RUNTIME_DIR" -maxdepth 1 -name 'wayland-*' ! -name '*.lock' 2>/dev/null | head -1 || true)"
+    [ -n "$sock" ] && break
+    sleep 0.5
+done
+[ -n "$sock" ] || { echo "::error::compositor did not create a Wayland socket"; exit 1; }
+export WAYLAND_DISPLAY="$(basename "$sock")"
+echo "compositor up on WAYLAND_DISPLAY=$WAYLAND_DISPLAY"
+echo "::endgroup::"
+
+# Give modules time to poll/settle.
+sleep 6
+
+echo "::group::Waybar log"
+cat "$LOG" || true
+echo "::endgroup::"
+
+# --- Level 1: smoke ---
+if ! pgrep -x waybar >/dev/null; then
+    echo "::error::waybar is not running — it crashed on startup (config: $CONFIG)"
+    exit 1
+fi
+if grep -iqE 'critical|segfault|\bfatal\b|terminate called' "$LOG"; then
+    echo "::error::waybar logged a fatal error (config: $CONFIG)"
+    exit 1
+fi
+echo "✓ waybar launched and stayed alive under headless sway"
+
+# --- Level 2: screenshot ---
+if [ -n "$SHOT" ]; then
+    grim "$SHOT"
+    echo "✓ captured screenshot -> $SHOT"
+fi