Add headless regression tests for Phase 4b features

- Implement test for popup anchor behavior in rule-builder menus to ensure consistent anchor positioning during menu transitions.
- Create tests for stage logic, including mode transitions, toolbar visibility, and status bar updates.
- Add terrain drag-painting tests to verify correct block placement behavior and conflict handling.
- Introduce walk waypoint tests to check for arrival conditions and position stability after navigation.

scripts/prop_library.gd

@@ -0,0 +1,50 @@
+class_name PropLibrary
+extends RefCounted
+
+const PROP_UTILS := preload("res://scripts/prop_utils.gd")
+const PROP_BLOCK := preload("res://scripts/prop_block.gd")
+
+static var _templates: Array[Dictionary] = []
+
+
+static func get_entries() -> Array[Dictionary]:
+	if _templates.is_empty():
+		_build_templates()
+	return _templates
+
+
+static func get_ids() -> Array[String]:
+	var ids: Array[String] = []
+	for t: Dictionary in get_entries():
+		ids.append(String(t["id"]))
+	return ids
+
+
+static func get_entry(id: String) -> Dictionary:
+	for t: Dictionary in get_entries():
+		if String(t["id"]) == id:
+			return t
+	return {}
+
+
+static func get_default_id() -> String:
+	return "crate"
+
+
+static func _build_templates() -> void:
+	_templates = [
+		_make("crate", "Crate", PROP_BLOCK.MaterialPreset.WOOD, "Wood", PROP_UTILS.create_box()),
+		_make("ball", "Ball", PROP_BLOCK.MaterialPreset.RUBBER, "Rubber", PROP_UTILS.create_ball()),
+		_make("plank", "Plank", PROP_BLOCK.MaterialPreset.METAL, "Metal", PROP_UTILS.create_plank()),
+		_make("triangle", "Triangle", PROP_BLOCK.MaterialPreset.CARDBOARD, "Cardboard", PROP_UTILS.create_triangle()),
+	]
+
+
+static func _make(id: String, name: String, preset: int, label: String, payload: Dictionary) -> Dictionary:
+	return {
+		"id": id,
+		"name": name,
+		"material_preset": preset,
+		"material_label": label,
+		"payload": payload,
+	}