feat: introduce v2.0 abstraction layer with core types, backend interface, and plugin system

feat: add fade effects with API control and real-time event subscription
test: add API integration tests with isolated test environment and coverage reporting
build: update Makefile for recursive source finding, dynamic linking, and test targets
docs: document abstraction layer, fade effects, and API enhancements in README

include/backends/x11/x11_backend.h

@@ -0,0 +1,35 @@
+/*
+ * DWN - Desktop Window Manager
+ * X11 Backend Header
+ */
+
+#ifndef X11_BACKEND_H
+#define X11_BACKEND_H
+
+#include "backends/backend_interface.h"
+
+#include <X11/Xlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*==============================================================================
+ * X11 Backend Access
+ *============================================================================*/
+
+const BackendInterface* x11_backend_get_interface(void);
+Display* x11_backend_get_display(void);
+Window x11_backend_get_root(void);
+int x11_backend_get_screen(void);
+
+static inline bool x11_backend_is_available(void) {
+    const BackendInterface *iface = x11_backend_get_interface();
+    return iface && iface->init(NULL);
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* X11_BACKEND_H */