Extract canvas document IO and preview pass setup

docs/modernization/tasks.md

@@ -91,7 +91,7 @@ Status: In Progress
 
 Why now:
 `src/canvas.cpp` is still the biggest single architectural blocker at about
-3622 lines.
+2645 lines.
 
 Current slice:
 - Canvas state-management helpers for picking, clear/clear-all, layer
@@ -99,6 +99,10 @@ Current slice:
   save/restore now live in `src/legacy_canvas_state_services.cpp` instead of
   staying inline in `src/canvas.cpp`, but the file still owns the larger
   document-plus-render shell.
+- Canvas import/export/save/open/thumbnail ownership now lives in
+  `src/legacy_canvas_document_io_services.cpp` instead of staying inline in
+  `src/canvas.cpp`, which materially reduces document IO ownership in the live
+  render shell.
 
 Write scope:
 - `src/canvas.cpp`
@@ -138,6 +142,12 @@ Current slice:
 - `NodeStrokePreview` final composite plus preview-texture copy now route
   through `legacy_node_stroke_preview_execution_services.h`, but the preview
   node still owns most live-pass and retained GL resource execution.
+- `NodeStrokePreview` render-target setup plus immediate-pass sequence
+  orchestration now also route through
+  `legacy_node_stroke_preview_execution_services.h`, and duplicate render-
+  target setup was removed from `render_to_image()` and the queued worker path,
+  but the preview node still owns broader live-pass state and thread-facing
+  orchestration.
 - `NodeCanvas` merged-path per-plane merged-texture draw execution now also
   routes through `execute_legacy_canvas_draw_merge_layer_texture(...)`.
 - `NodeCanvas` merged-path and non-blend checkerboard background setup now also