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

manual/index.html

@@ -13,7 +13,7 @@
         <aside class="sidebar">
             <div class="sidebar-header">
                 <h1>DWN</h1>
-                <span class="version">v1.0.0</span>
+                <span class="version">v2.0.0</span>
             </div>
 
             <div class="search-box">
@@ -47,6 +47,8 @@
                 <div class="nav-section">
                     <div class="nav-section-title">Advanced</div>
                     <a href="architecture.html" class="nav-link">Architecture</a>
+                    <a href="abstraction-layer.html" class="nav-link">Abstraction Layer</a>
+                    <a href="plugin-development.html" class="nav-link">Plugin Development</a>
                     <a href="building.html" class="nav-link">Building from Source</a>
                 </div>
             </nav>
@@ -56,7 +58,7 @@
             <div class="content">
                 <div class="hero">
                     <h1>DWN</h1>
-                    <p class="tagline">A modern X11 window manager with AI integration and WebSocket API</p>
+                    <p class="tagline">A modern X11 window manager with AI integration, WebSocket API, and plugin extensibility</p>
                     <div class="btn-group">
                         <a href="installation.html" class="btn">Get Started</a>
                         <a href="api-overview.html" class="btn btn-outline">API Docs</a>
@@ -67,7 +69,7 @@
                     <div class="feature-card">
                         <div class="feature-icon">🪟</div>
                         <div class="feature-title">Multiple Layouts</div>
-                        <div class="feature-desc">Tiling, floating, and monocle layouts with per-workspace configuration. Master-stack tiling with adjustable ratios.</div>
+                        <div class="feature-desc">Tiling, floating, monocle, and grid layouts with per-workspace configuration. Plugin API for custom layouts.</div>
                     </div>
                     <div class="feature-card">
                         <div class="feature-icon">🖥️</div>
@@ -94,6 +96,16 @@
                         <div class="feature-title">Notifications</div>
                         <div class="feature-desc">Built-in D-Bus notification daemon with configurable appearance.</div>
                     </div>
+                    <div class="feature-card">
+                        <div class="feature-icon">🔧</div>
+                        <div class="feature-title">Plugin System</div>
+                        <div class="feature-desc">Extensible plugin architecture for layouts and widgets. Dynamic loading support.</div>
+                    </div>
+                    <div class="feature-card">
+                        <div class="feature-icon">📐</div>
+                        <div class="feature-title">Abstraction Layer</div>
+                        <div class="feature-desc">Backend-agnostic architecture. Ready for X11, Wayland, and headless backends.</div>
+                    </div>
                 </div>
 
                 <div class="quick-start">