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

build/main.d

@@ -19,9 +19,12 @@ build/main.o: src/main.c include/dwn.h include/config.h include/dwn.h \
  /usr/include/dbus-1.0/dbus/dbus-signature.h \
  /usr/include/dbus-1.0/dbus/dbus-syntax.h \
  /usr/include/dbus-1.0/dbus/dbus-threads.h include/systray.h \
- include/news.h include/applauncher.h include/ai.h include/autostart.h \
- include/services.h include/api.h include/demo.h include/screenshot.h \
- include/ocr.h include/util.h
+ include/slider.h include/news.h include/applauncher.h include/ai.h \
+ include/autostart.h include/services.h include/api.h include/demo.h \
+ include/screenshot.h include/ocr.h include/util.h include/slider.h \
+ include/rules.h include/marks.h include/core/wm_client.h \
+ include/core/wm_types.h include/core/wm_string.h include/core/wm_types.h \
+ include/plugins/layout_plugin.h include/plugins/builtin_layouts.h
 include/dwn.h:
 include/config.h:
 include/dwn.h:
@@ -52,6 +55,7 @@ include/notifications.h:
 /usr/include/dbus-1.0/dbus/dbus-syntax.h:
 /usr/include/dbus-1.0/dbus/dbus-threads.h:
 include/systray.h:
+include/slider.h:
 include/news.h:
 include/applauncher.h:
 include/ai.h:
@@ -62,3 +66,12 @@ include/demo.h:
 include/screenshot.h:
 include/ocr.h:
 include/util.h:
+include/slider.h:
+include/rules.h:
+include/marks.h:
+include/core/wm_client.h:
+include/core/wm_types.h:
+include/core/wm_string.h:
+include/core/wm_types.h:
+include/plugins/layout_plugin.h:
+include/plugins/builtin_layouts.h: