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.

src/test/capture_prop_gallery.gd

@@ -1,11 +1,12 @@
 extends SceneTree
 ## Dev helper: render every generated prop side by side on a plain backdrop and
-## save user://screenshots/10-prop-gallery.png. The image feeds
+## save user://screenshots/10-prop-gallery.png by default. The image feeds
 ## `tools/screen_designs.py compare` so a prop regression is visible.
 ##
 ## Usage: godot --path . --script res://src/test/capture_prop_gallery.gd
+##        godot --path . --script res://src/test/capture_prop_gallery.gd -- --out=res://docs/screenshots
 
-const OUT_DIR := "user://screenshots"
+var out_dir := "user://screenshots"
 const MODEL_DIR := "res://assets/models/generated/"
 const LIBRARY_DIR := "res://assets/models/blender/props/"
 const PROPS := ["bollard", "bench", "trash_bin", "planter_box",
@@ -15,6 +16,9 @@ const SPACING := 1.9
 var frames := 0
 
 func _initialize() -> void:
+	for arg in OS.get_cmdline_user_args():
+		if arg.begins_with("--out="):
+			out_dir = arg.trim_prefix("--out=")
 	root.size = Vector2i(1280, 720)
 	_build_stage()
 
@@ -102,7 +106,7 @@ func _process(_delta: float) -> bool:
 	if dir == null or not dir.dir_exists("screenshots"):
 		dir.make_dir_recursive("screenshots")
 	var image := root.get_texture().get_image()
-	var err := image.save_png(OUT_DIR.path_join("10-prop-gallery.png"))
+	var err := image.save_png(out_dir.path_join("10-prop-gallery.png"))
 	print("10-prop-gallery.png -> err=%d (%dx%d)" % [err, image.get_width(), image.get_height()])
 	quit(0)
 	return true