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

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