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

docs/modernization/roadmap.md

@@ -83,11 +83,11 @@ Current hotspot files:
 - `src/app_layout.cpp`: 125 lines
 - `src/canvas_modes.cpp`: 402 lines
 - `src/node.cpp`: 389 lines
-- `src/main.cpp`: 271 lines
-- `src/node_panel_brush.cpp`: 435 lines
+- `src/main.cpp`: 166 lines
+- `src/node_panel_brush.cpp`: 337 lines
 - `src/node_stroke_preview.cpp`: 562 lines
 - `src/node_canvas.cpp`: 219 lines
-- `src/app.cpp`: 292 lines
+- `src/app.cpp`: 171 lines
 - `src/app_dialogs.cpp`: 168 lines
 
 Current architecture mismatches that must be treated as real blockers:
@@ -216,8 +216,13 @@ Current architecture mismatches that must be treated as real blockers:
   `src/platform_windows/windows_bootstrap_helpers.cpp` instead of
   `src/main.cpp`, while the Win32 window procedure and retained message-handling
   shell now also live in `src/platform_windows/windows_window_shell.*`
-  instead of `src/main.cpp`, which materially thins `src/main.cpp` even
-  though broader entrypoint/runtime composition still remains there,
+  instead of `src/main.cpp`, while the remaining interactive Win32 runtime
+  pocket for touch registration, render/UI thread startup, GL debug callback
+  hookup, Wintab initialization/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 materially thins `src/main.cpp` even though broader
+  entrypoint/runtime composition still remains there,
   while `App::rec_loop()` now delegates worker-iteration orchestration into
   the retained recording bridge, `App::update_rec_frames()` now delegates
   recording label refresh through that same retained recording path, and the

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.*`