Add document PPI export boundary

src/document/ppi_export.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "document/document.h"
+#include "foundation/result.h"
+
+#include <cstddef>
+#include <vector>
+
+namespace pp::document {
+
+[[nodiscard]] pp::foundation::Result<std::vector<std::byte>> export_ppi_project_document(
+    const CanvasDocument& document);
+
+}