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