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/client.d

@@ -19,7 +19,7 @@ build/client.o: src/client.c include/client.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/layout.h \
- include/panel.h include/api.h
+ include/panel.h include/api.h include/rules.h include/marks.h
 include/client.h:
 include/dwn.h:
 include/atoms.h:
@@ -49,3 +49,5 @@ include/notifications.h:
 include/layout.h:
 include/panel.h:
 include/api.h:
+include/rules.h:
+include/marks.h:

build/keys.d

@@ -19,7 +19,7 @@ build/keys.o: src/keys.c include/keys.h include/dwn.h include/client.h \
  /usr/include/dbus-1.0/dbus/dbus-syntax.h \
  /usr/include/dbus-1.0/dbus/dbus-threads.h include/news.h \
  include/applauncher.h include/decorations.h include/demo.h \
- include/layout.h include/api.h
+ include/layout.h include/api.h include/marks.h
 include/keys.h:
 include/dwn.h:
 include/client.h:
@@ -52,3 +52,4 @@ include/decorations.h:
 include/demo.h:
 include/layout.h:
 include/api.h:
+include/marks.h:

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:

build/panel.d

@@ -1,6 +1,7 @@
 build/panel.o: src/panel.c include/panel.h include/dwn.h \
  include/workspace.h include/layout.h include/client.h include/config.h \
- include/util.h include/atoms.h include/systray.h include/news.h
+ include/util.h include/atoms.h include/systray.h include/slider.h \
+ include/news.h
 include/panel.h:
 include/dwn.h:
 include/workspace.h:
@@ -10,4 +11,5 @@ include/config.h:
 include/util.h:
 include/atoms.h:
 include/systray.h:
+include/slider.h:
 include/news.h:

build/systray.d

@@ -1,6 +1,6 @@
 build/systray.o: src/systray.c include/systray.h include/dwn.h \
- include/panel.h include/config.h include/util.h include/notifications.h \
- /usr/include/dbus-1.0/dbus/dbus.h \
+ include/slider.h include/panel.h include/config.h include/util.h \
+ include/notifications.h /usr/include/dbus-1.0/dbus/dbus.h \
  /usr/lib/x86_64-linux-gnu/dbus-1.0/include/dbus/dbus-arch-deps.h \
  /usr/include/dbus-1.0/dbus/dbus-macros.h \
  /usr/include/dbus-1.0/dbus/dbus-address.h \
@@ -17,9 +17,10 @@ build/systray.o: src/systray.c include/systray.h include/dwn.h \
  /usr/include/dbus-1.0/dbus/dbus-server.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/atoms.h
+ /usr/include/dbus-1.0/dbus/dbus-threads.h include/atoms.h include/api.h
 include/systray.h:
 include/dwn.h:
+include/slider.h:
 include/panel.h:
 include/config.h:
 include/util.h:
@@ -43,3 +44,4 @@ include/notifications.h:
 /usr/include/dbus-1.0/dbus/dbus-syntax.h:
 /usr/include/dbus-1.0/dbus/dbus-threads.h:
 include/atoms.h:
+include/api.h: