Route live save snapshots through PPI policy

src/app_core/document_canvas.h

@@ -78,6 +78,19 @@ struct DocumentCanvasSaveSnapshotReport {
     bool can_export_ppi = false;
 };
 
+enum class DocumentCanvasSaveWriterAction {
+    use_document_ppi_writer,
+    use_legacy_project_save,
+};
+
+struct DocumentCanvasSaveWriterRoutePlan {
+    DocumentCanvasSaveWriterAction action = DocumentCanvasSaveWriterAction::use_legacy_project_save;
+    bool payload_complete = false;
+    bool can_export_ppi = false;
+    bool uses_document_ppi_writer = false;
+    std::string_view fallback_reason;
+};
+
 struct DocumentCanvasPpiExportResult {
     DocumentCanvasSaveSnapshotReport report;
     std::vector<std::byte> bytes;
@@ -248,14 +261,31 @@ public:
     };
 }
 
+[[nodiscard]] constexpr DocumentCanvasSaveWriterRoutePlan plan_document_canvas_save_writer_route(
+    DocumentCanvasSaveSnapshotReport report) noexcept
+{
+    DocumentCanvasSaveWriterRoutePlan plan;
+    plan.payload_complete = report.payload_complete;
+    plan.can_export_ppi = report.can_export_ppi;
+
+    if (!report.payload_complete || !report.can_export_ppi) {
+        plan.fallback_reason = "canvas document snapshot still requires renderer payload readback";
+        return plan;
+    }
+
+    plan.action = DocumentCanvasSaveWriterAction::use_document_ppi_writer;
+    plan.uses_document_ppi_writer = true;
+    return plan;
+}
+
 [[nodiscard]] inline pp::foundation::Result<DocumentCanvasPpiExportResult>
 export_document_canvas_save_snapshot_to_ppi(const DocumentCanvasSnapshotResult& snapshot)
 {
     const auto report = make_document_canvas_save_snapshot_report(snapshot);
-    if (!report.can_export_ppi) {
+    const auto route = plan_document_canvas_save_writer_route(report);
+    if (!route.uses_document_ppi_writer) {
         return pp::foundation::Result<DocumentCanvasPpiExportResult>::failure(
-            pp::foundation::Status::invalid_argument(
-                "canvas document snapshot still requires renderer payload readback"));
+            pp::foundation::Status::invalid_argument(route.fallback_reason.data()));
     }
 
     auto bytes = pp::document::export_ppi_project_document(snapshot.document);