Extract brush panel UI, app startup shell, and Win32 runtime shell

docs/modernization/tasks.md

@@ -536,7 +536,7 @@ Status: In Progress
 
 Why now:
 `src/app.cpp` still carries startup, frame flow, queue draining, recording,
-and composition logic in one 292-line file.
+and composition logic in one 171-line file.
 
 Current slice:
 - UI observer math now routes through `src/legacy_app_frame_services.cpp`
@@ -553,6 +553,11 @@ Current slice:
   `App::render_thread_tick(...)` now also live in
   `src/legacy_app_runtime_shell_services.cpp` instead of staying inline in
   `src/app.cpp`, which materially thins the remaining frame/runtime shell.
+- `App::create(...)`, `App::initAssets(...)`, `App::initLog(...)`, and
+  `App::init(...)` now also live in `src/legacy_app_startup_services.*`
+  instead of staying inline in `src/app.cpp`, which reduces the remaining app
+  file to a thinner retained composition surface around startup and runtime
+  delegation.
 
 Write scope:
 - `src/app.cpp`
@@ -639,6 +644,12 @@ Current slice:
   in `src/platform_windows/windows_window_shell.*` instead of `src/main.cpp`,
   which materially thins the entry file even though broader runtime/entrypoint
   composition is still open
+- the remaining interactive Win32 runtime pocket for touch registration,
+  render/UI thread startup, debug GL callback hookup, Wintab init/skip, icon
+  setup, placement restore, optional VR start, splash dismissal, message
+  loop, and shutdown cleanup now also lives in
+  `src/platform_windows/windows_runtime_shell.*` instead of `src/main.cpp`,
+  which reduces the entry TU to a much smaller composition root
 - prepared-file background work now runs through an `AppRuntime`-owned worker
   queue instead of a retained static worker in `src/app_events.cpp`
 - canvas async import/export/save/open background work now also runs through an
@@ -1089,6 +1100,12 @@ Current slice:
   `src/node_panel_brush.cpp`, which materially thins another retained preset
   panel UI pocket even though cloud/package worker ownership remains the
   follow-up.
+- `NodePanelBrush` brush texture panel init, selection dispatch,
+  popup-close event handling, restore-failure prompt flow, and added-state
+  reset now also live in `src/legacy_brush_panel_ui.*` instead of staying
+  inline in `src/node_panel_brush.cpp`, which trims another retained brush UI
+  workflow pocket while preserving the live node as a thinner controller
+  surface.
 
 Write scope:
 - `src/legacy_cloud_services.*`