Move app open routing into document helper

docs/modernization/tasks.md

@@ -341,6 +341,15 @@ Current slice:
   `handle_legacy_node_canvas_event(...)` in
   `src/legacy_canvas_tool_services.*`, which moves the live input/controller
   pocket out of `src/node_canvas.cpp` and keeps the node on a thinner shell.
+- `App::dialog_layer_rename()` now routes through
+  `open_legacy_document_layer_rename_dialog(...)` in
+  `src/legacy_document_layer_services.*`, which moves the remaining dialog
+  overlay-open/wire/close workflow out of `src/app_dialogs.cpp` and leaves
+  the app dialog shell thinner.
+- `App::open_document(...)` now routes through
+  `execute_legacy_document_open(...)` in `src/legacy_document_open_services.*`,
+  which moves route classification and unsaved-project gating out of
+  `src/app.cpp` and into the retained document-open helper.
 - `NodeCanvas` restore/clear context, resize handling, camera reset, buffer
   creation, cursor visibility/update, tick, and destroy ownership now also
   live in `src/legacy_node_canvas_state_services.*` instead of staying inline
@@ -385,6 +394,10 @@ Current slice:
   now also lives in `src/legacy_ui_node_style.*` instead of staying inline in
   `src/node.cpp`, which trims another coherent base scene-graph shell pocket
   without changing the public surface.
+- `NodePanelBrushPreset` global panel registration now also lives in
+  `src/legacy_brush_preset_list_services.*` instead of staying on the live
+  node type as a static registry field, which trims another retained
+  controller-state pocket from `src/node_panel_brush.cpp`.
 
 Write scope:
 - `src/node_stroke_preview.cpp`
@@ -533,7 +546,7 @@ Status: In Progress
 
 Why now:
 `src/app_dialogs.cpp` still mixes document workflow decisions, export routing,
-dialog construction, and overlay ownership in one 168-line shell.
+dialog construction, and overlay ownership in one 106-line shell.
 
 Current slice:
 - Informational overlay opener paths for user manual, changelog, about,
@@ -546,8 +559,15 @@ Current slice:
   entrypoints are now thin call-throughs, but new/open/save/browse/resize and
   retained dialog execution are still inline in `src/app_dialogs.cpp`.
 - New/open/save/browse/resize workflow entrypoints now also live in
-  `src/app_dialogs_workflow.cpp`, and `src/app_dialogs.cpp` is down to the
-  remaining thin entrypoints plus layer-rename retained dialog glue.
+  `src/app_dialogs_workflow.cpp`, and the layer-rename dialog open / wire /
+  close pocket now lives in `src/legacy_document_layer_services.*`, leaving
+  `src/app_dialogs.cpp` down to the remaining thin entrypoints plus other
+  retained dialog glue.
+- `App::open_document()` now routes document classification and unsaved-
+  project gating through `src/legacy_document_open_services.cpp` instead of
+  keeping that shell pocket inline in `src/app.cpp`, which trims the live app
+  entry file a little further while preserving the retained open-action
+  behavior.
 
 Write scope:
 - `src/app_dialogs.cpp`