Add a repeatable screenshot capture and compare smoke

Both capture helpers now accept --out=res://docs/screenshots, and
tools/check_screenshots.sh renders the staged set (using xvfb-run when headless)
before running screen_designs.py compare. Record the judge result: 7 match,
4 review, 0 diff. Headless 226/226 and the smoke both pass in a clean worktree.

tools/check_screenshots.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Repeatable visual-regression smoke: render the staged captures into
+# docs/screenshots and compare them against the runtime reference art.
+#
+# Runs under Xvfb automatically when no display is available. Requires Godot
+# 4.3 on PATH (or GODOT=...), Python 3, Pillow, and xvfb-run for headless hosts.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+GODOT="${GODOT:-godot}"
+RUN=()
+if [[ -z "${DISPLAY:-}" ]]; then
+  if ! command -v xvfb-run >/dev/null 2>&1; then
+    echo "error: no DISPLAY and xvfb-run is not installed" >&2
+    exit 2
+  fi
+  RUN=(xvfb-run -a)
+fi
+
+"${RUN[@]}" "$GODOT" --path . --script res://src/test/capture_screenshots.gd -- --out=res://docs/screenshots
+"${RUN[@]}" "$GODOT" --path . --script res://src/test/capture_prop_gallery.gd -- --out=res://docs/screenshots
+python3 tools/screen_designs.py compare