Own Win32 startup flow in runtime layer

docs/modernization/tasks.md

@@ -57,7 +57,8 @@ Completed, blocked, and superseded task history moved to
     now also binds directly from `src/platform_web/web_platform_services.*`,
     but the broader non-Windows fallback adapter still exists as generic
     fallback policy
-  - `platform_legacy` is still part of the live app shell
+  - the deleted `platform_legacy` shim is no longer in the root app graph, but
+    retained package/build compatibility edges still need cleanup
 - The app runtime boundary is not finished:
   - render/UI queues are static `App` state
   - app-facing detached launches are no longer the main issue; preview and
@@ -66,12 +67,13 @@ Completed, blocked, and superseded task history moved to
   - canvas async import/export/save/open now run through an owned in-file
     worker, but their retained progress execution is still not a clean runtime
     service boundary
-  - the Win32 input path now binds the active `WacomTablet*` explicitly
-    through `windows_runtime_shell`, but that tablet binding still lives at a
+  - the Win32 input path now binds the active `WacomTablet*` through
+    `windows_runtime_state.*`, but that tablet binding still lives at a
     composition edge instead of a broader runtime/platform-owned controller
-  - the touched Win32 app/window/session queries now route through narrow
-    runtime-shell accessors, but the broader runtime/thread host still owns
-    composition-edge global state and shutdown sequencing
+  - the touched Win32 startup/session flow now routes through
+    `windows_runtime_flow.*` instead of `windows_runtime_shell.cpp`, but the
+    broader runtime/thread host still owns composition-edge global state and
+    shutdown sequencing
     - thread-affinity rules are enforced by convention and asserts instead of
       explicit runtime contracts
 - The UI ownership boundary is not finished:
@@ -84,6 +86,16 @@ Completed, blocked, and superseded task history moved to
   the queue is now ordered by code movement instead.
 
 Current slice:
+- `src/platform_windows/windows_runtime_flow.*` now owns the live Win32
+  startup/session composition flow instead of leaving that body in
+  `src/platform_windows/windows_runtime_shell.cpp`.
+- A bound runtime-flow owner now binds the live `MainWindowSession`, runs the
+  Win32 preflight startup steps, handles the existing `convert` / `-vrmode`
+  mode switch, and tears down the bound app/session through one destructor
+  path.
+- `src/platform_windows/windows_runtime_shell.cpp` is down to a thin entry
+  wrapper over `run_main_application_flow(...)`, so the repeated app/session
+  teardown branches are gone from the shell.
 - The orphaned `src/platform_legacy/legacy_platform_services.*` shim and
   `src/platform_legacy/legacy_platform_fallback_behavior.h` helper are now
   deleted.