fix: guard fsPath underflow, name WMO doodad mask, add why-comments

- asset_manager: add size guard before fsPath.substr(size-4) in
  tryLoadPngOverride — resolveFile could theoretically return a
  path shorter than the extension
- wmo_loader: name kDoodadNameIndexMask (0x00FFFFFF) with why-comment
  explaining the 24-bit name index / 8-bit flags packing and MODN
  string table reference
- window: add why-comment on LOG_WARNING usage during shutdown —
  intentionally elevated so teardown progress is visible at default
  log levels for crash diagnosis

src/core/window.cpp

@@ -103,6 +103,8 @@ bool Window::initialize() {
     return true;
 }
 
+// Shutdown progress uses LOG_WARNING so these messages are always visible even at
+// default log levels — useful for diagnosing hangs or crashes during teardown.
 void Window::shutdown() {
     LOG_WARNING("Window::shutdown - vkContext...");
     if (vkContext) {