Use Win32 runtime binding for thread control

docs/modernization/roadmap.md

@@ -70,6 +70,10 @@ What is already real:
 - `pp_app_core`
 
 Latest slice:
+- `src/platform_windows/windows_runtime_shell.cpp` and
+  `src/platform_windows/windows_lifecycle_shell.cpp` now drive thread
+  start/stop through the explicit Windows runtime binding instead of routing
+  that control through `App::runtime()`.
 - `src/platform_windows/windows_platform_services.cpp` no longer brokers the
   Win32 main-thread queue through `bound_app()->runtime()`; it now uses an
   explicit Windows runtime binding.

docs/modernization/tasks.md

@@ -78,6 +78,9 @@ Completed, blocked, and superseded task history moved to
   the queue is now ordered by code movement instead.
 
 Current slice:
+- `src/platform_windows/windows_runtime_shell.cpp` and
+  `src/platform_windows/windows_lifecycle_shell.cpp` now use the explicit
+  Windows runtime binding for thread start/stop control.
 - `src/platform_windows/windows_platform_services.cpp` no longer uses
   `bound_app()->runtime()` for main-thread task brokering.
 - `src/platform_windows/windows_runtime_state.*` now carries the active

src/platform_windows/windows_lifecycle_shell.cpp

@@ -7,6 +7,7 @@
 #include "platform_windows/windows_lifecycle_state.h"
 #include "platform_windows/windows_main_window_session.h"
 #include "platform_windows/windows_runtime_shell.h"
+#include "platform_windows/windows_runtime_state.h"
 #include "platform_windows/windows_stylus_input.h"
 
 namespace pp::platform::windows {
@@ -19,10 +20,14 @@ void request_window_close(HWND hWnd)
 void handle_window_close_message(VrShellState& vr)
 {
     auto* app = bound_app();
+    auto* runtime = retained_bound_runtime();
     mark_lifecycle_stopped();
     request_stop_and_join_vr_thread(vr);
-    app->runtime().ui_thread_stop();
-    app->runtime().render_thread_stop();
+    if (runtime)
+    {
+        runtime->ui_thread_stop();
+        runtime->render_thread_stop();
+    }
     app->terminate();
     release_bound_app();
     PostQuitMessage(0);

src/platform_windows/windows_runtime_shell.cpp

@@ -30,9 +30,11 @@ void register_touch_window(HWND hWnd)
 void initialize_runtime_threads()
 {
     auto* app = bound_app();
+    auto* runtime = retained_bound_runtime();
+    assert(runtime);
     mark_lifecycle_running();
-    app->runtime().render_thread_start(*app);
-    app->runtime().ui_thread_start(*app);
+    runtime->render_thread_start(*app);
+    runtime->ui_thread_start(*app);
 }
 
 void install_debug_gl_callbacks()