Add source-generated Mediator pattern with zero runtime dependency + comprehensive production demo

docs/patterns/messaging/README.md

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+# Messaging Patterns
+
+This section covers messaging and event-driven patterns in PatternKit, with a focus on the **Mediator pattern**.
+
+## Mediator (Source Generated)
+
+A **zero-dependency**, **source-generated Mediator pattern** implementation for commands, notifications, and streams.
+
+The **Mediator pattern** reduces coupling between components by centralizing communication through a mediator object. This source-generated variant provides compile-time code generation with zero runtime dependencies on PatternKit.
+
+[Learn More →](dispatcher.md)
+
+### Quick Start
+
+```csharp
+using PatternKit.Generators.Messaging;
+
+// Mark assembly for generation
+[assembly: GenerateDispatcher(
+    Namespace = "MyApp.Messaging",
+    Name = "AppDispatcher")]
+
+// Define messages
+public record CreateUser(string Username, string Email);
+public record UserCreated(int UserId, string Username);
+
+// Build mediator
+var dispatcher = AppDispatcher.Create()
+    .Command<CreateUser, UserCreated>((req, ct) =>
+        new ValueTask<UserCreated>(new UserCreated(1, req.Username)))
+    .Build();
+
+// Use mediator
+var result = await dispatcher.Send<CreateUser, UserCreated>(
+    new CreateUser("alice", "alice@example.com"),
+    cancellationToken);
+```
+
+### Key Features
+
+- ✅ **Zero runtime dependency** on PatternKit
+- ✅ **AOT-friendly** - no reflection
+- ✅ **Async-first** - ValueTask & IAsyncEnumerable<T>
+- ✅ **Commands** - request → response
+- ✅ **Notifications** - fan-out to multiple handlers
+- ✅ **Streams** - async enumerable results
+- ✅ **Pipelines** - pre/post hooks for cross-cutting concerns
+- ✅ **Fluent API** - easy to compose
+
+### Documentation
+
+- [Full Documentation](dispatcher.md)
+- [Simple Examples](../../src/PatternKit.Examples/Messaging/DispatcherExample.cs)
+- **[Comprehensive Production Demo](../../src/PatternKit.Examples/MediatorComprehensiveDemo/ComprehensiveDemo.cs)** - DI integration, CQRS, behaviors, real-world domain
+
+### Related Patterns
+
+The Source-Generated Mediator complements other PatternKit patterns:
+
+- **[Runtime Mediator](../behavioral/mediator/index.md)** - Pre-built mediator with PatternKit runtime (use for application code)
+- **Observer** - For reactive event handling and pub/sub
+- **Command** - For encapsulating requests as objects
+
+### When to Use
+
+Use the source-generated Mediator when you need:
+
+- ✅ Decoupled message handling in your application
+- ✅ Compile-time verification of message flows
+- ✅ Zero runtime dependencies (for libraries/NuGet packages)
+- ✅ AOT deployment scenarios
+- ✅ High-performance message routing without reflection
+
+### When Not to Use
+
+Consider alternatives when:
+
+- ❌ You need distributed messaging (use message broker instead)
+- ❌ You need dynamic handler discovery at runtime
+- ❌ Your handlers come from plugins/dynamic assemblies
+- ❌ You need complex routing logic (use message broker)