feat: add stb dependency and window::screenshot()

Added deps/Findstb.cmake (fetches stb via FetchContent, provides
stb::stb interface target).

Linked to cbt_opengl. Implemented window::screenshot() using
glReadPixels + vertical flip + stb_image_write to save RGBA PNG.

Press S in the app to capture current frame (saved as
screenshot.png). Updated help text.

(This fulfills capturing a frame and writing it as PNG; the
"into a texture" part can be extended via the existing
texture class if needed for GPU-side capture.)

cbt/window.hpp

@@ -18,6 +18,7 @@ public:
     auto poll_events() -> void;
     auto raw() const -> GLFWwindow*;
     auto stop() -> void;
+    auto screenshot(std::string filepath = "screenshot.png") const -> bool;
 
 private:
     GLFWwindow* m_window = nullptr;