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.cpp

@@ -3,6 +3,8 @@
 #include <windows.h>
 #endif
 
+#include <vector>
+
 #define GLFW_INCLUDE_NONE
 #include "GLFW/glfw3.h"
 
@@ -12,6 +14,9 @@
 #endif
 
 #include "fmt/std.h"
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#include "stb_image_write.h"
+#include "glad/glad.h"
 
 #include "cbt/window.hpp"
 
@@ -89,4 +94,37 @@ auto window::stop() -> void {
     }
 }
 
+auto window::screenshot(std::string filepath) const -> bool {
+    if (!m_window || !valid()) {
+        return false;
+    }
+
+    int width = 0;
+    int height = 0;
+    glfwGetFramebufferSize(m_window, &width, &height);
+    if (width <= 0 || height <= 0) {
+        return false;
+    }
+
+    std::vector<unsigned char> pixels(static_cast<size_t>(width * height * 4));
+    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels.data());
+
+    // Flip vertically (OpenGL reads bottom-up; PNG is top-down)
+    for (int y = 0; y < height / 2; ++y) {
+        int y2 = height - 1 - y;
+        for (int x = 0; x < width * 4; ++x) {
+            std::swap(pixels[static_cast<size_t>(y * width * 4 + x)],
+                      pixels[static_cast<size_t>(y2 * width * 4 + x)]);
+        }
+    }
+
+    if (stbi_write_png(filepath.c_str(), width, height, 4, pixels.data(), width * 4) != 0) {
+        fmt::print("Screenshot saved to {}\n", filepath);
+        return true;
+    }
+
+    fmt::print("Failed to write screenshot to {}\n", filepath);
+    return false;
+}
+
 }

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;