Extract app frame and export dialog helpers

docs/modernization/tasks.md

@@ -199,7 +199,12 @@ Current slice:
 - `NodeCanvas` non-`draw_merged` per-layer temporary erase/paint, layer-texture,
   and blend setup now also route through
   `make_legacy_canvas_draw_merge_layer_path_gl_execution(...)`, but the node
-  still owns the remaining draw lambdas and broader renderer-state shell.
+  still owned the remaining draw lambdas and broader renderer-state shell.
+- `NodeCanvas` now routes that remaining non-`draw_merged` per-layer blend,
+  composite, debug-outline, and frame callback assembly through the local
+  `make_node_canvas_layer_path_execution(...)` helper, which materially thins
+  `NodeCanvas::draw()` even though the broader draw loop still lives in
+  `src/node_canvas.cpp`.
 
 Write scope:
 - `src/node_stroke_preview.cpp`
@@ -322,7 +327,7 @@ Mini-model packet:
 
 #### ARC-APP-002 - Split `app_dialogs.cpp` Into Workflow Adapters And Widget Openers
 
-Status: Ready
+Status: In Progress
 
 Why now:
 `src/app_dialogs.cpp` still mixes document workflow decisions, export routing,
@@ -334,6 +339,10 @@ Current slice:
   and the corresponding `App::dialog_*` entrypoints are now thin call-throughs,
   but document/export workflow and retained dialog execution are still inline in
   `src/app_dialogs.cpp`.
+- Export, video-export, and PPBR export entrypoints now also live in
+  `src/app_dialogs_export.cpp`, and the corresponding `App::dialog_*`
+  entrypoints are now thin call-throughs, but new/open/save/browse/resize and
+  retained dialog execution are still inline in `src/app_dialogs.cpp`.
 
 Write scope:
 - `src/app_dialogs.cpp`
@@ -364,11 +373,19 @@ Mini-model packet:
 
 #### ARC-APP-003 - Reduce `app.cpp` To Frame, Queue, And Composition Shell
 
-Status: Ready
+Status: In Progress
 
 Why now:
 `src/app.cpp` still carries startup, frame flow, queue draining, recording,
-observer math, and composition logic in one 950-line file.
+and composition logic in one 502-line file.
+
+Current slice:
+- UI observer math now routes through `src/legacy_app_frame_services.cpp`
+  instead of staying inline in `src/app.cpp`.
+- The repeated UI child traversal in `App::draw()` now routes through the same
+  retained helper.
+- Canvas toolbar refresh in `App::update()` now also routes through that helper
+  file, materially shrinking `src/app.cpp`.
 
 Write scope:
 - `src/app.cpp`