fix(diagnostics): add render-phase crash markers and improve signal handling

Add signal-safe render-phase markers throughout GameScreen::render() and
Application::render() so the crash handler can report which render call was
active when a SIGSEGV occurs. The AMD RADV crash backtrace only shows 2
frames due to missing frame pointers, making it impossible to identify the
actual crash site.

Changes:
- Add volatile g_crashRenderPhase marker updated before each major render call
- Upgrade Linux signal handler to sigaction with SA_SIGINFO for faulting address
- Set ImGui CheckVkResultFn to log silent Vulkan errors in ImGui backend
- Enable -fno-omit-frame-pointer in all build configs (not just Debug/RelWithDebInfo)

src/core/application.cpp

@@ -2133,15 +2133,22 @@ void Application::update(float deltaTime) {
     }
 }
 
+// Render-phase marker from game_screen.cpp — updated here for 3D/submit phases
+} } // close wowee::core temporarily
+extern volatile const char* g_crashRenderPhase;
+namespace wowee { namespace core {
+
 void Application::render() {
     if (!renderer) {
         return;
     }
 
+    g_crashRenderPhase = "beginFrame";
     renderer->beginFrame();
 
     // Only render 3D world when in-game
     if (state == AppState::IN_GAME) {
+        g_crashRenderPhase = "renderWorld";
         if (world) {
             renderer->renderWorld(world.get(), gameHandler.get());
         } else {
@@ -2151,15 +2158,19 @@ void Application::render() {
 
     // Render performance HUD (within ImGui frame, before UI ends the frame)
     if (renderer) {
+        g_crashRenderPhase = "renderHUD";
         renderer->renderHUD();
     }
 
     // Render UI on top (ends ImGui frame with ImGui::Render())
     if (uiManager) {
+        g_crashRenderPhase = "uiRender";
         uiManager->render(state, authHandler.get(), gameHandler.get());
     }
 
+    g_crashRenderPhase = "endFrame";
     renderer->endFrame();
+    g_crashRenderPhase = "idle";
 }
 
 void Application::setupUICallbacks() {