Extract canvas document IO and preview pass setup

docs/modernization/roadmap.md

@@ -79,13 +79,13 @@ What is still carrying too much live ownership:
 
 Current hotspot files:
 
-- `src/canvas.cpp`: 3622 lines
+- `src/canvas.cpp`: 2645 lines
 - `src/app_layout.cpp`: 1498 lines
 - `src/canvas_modes.cpp`: 1798 lines
 - `src/node.cpp`: 1551 lines
 - `src/main.cpp`: 1374 lines
 - `src/node_panel_brush.cpp`: 1197 lines
-- `src/node_stroke_preview.cpp`: 1129 lines
+- `src/node_stroke_preview.cpp`: 933 lines
 - `src/node_canvas.cpp`: 888 lines
 - `src/app.cpp`: 950 lines
 - `src/app_dialogs.cpp`: 908 lines
@@ -155,8 +155,11 @@ Current architecture mismatches that must be treated as real blockers:
   canvas state-management cluster for picking, clear/clear-all, layer
   add/remove/order/lookups, animation frame control, resize, and snapshot
   save/restore now lives in `src/legacy_canvas_state_services.cpp` instead of
-  `src/canvas.cpp`, even though the bridge still owns worker-side readback flow
-  and encoder-state label reads.
+  `src/canvas.cpp`, while the larger import/export/save/open/thumbnail
+  document-IO cluster now lives in `src/legacy_canvas_document_io_services.cpp`
+  and `NodeStrokePreview` render-target setup plus immediate-pass sequencing
+  now route through retained preview execution helpers, even though the bridge
+  still owns worker-side readback flow and encoder-state label reads.
 - Modern C++23 usage exists in extracted components, especially `std::span`,
   explicit result/status objects, and a few concepts, but the live app still
   does not consistently express ownership, thread affinity, or renderer