[Feature]: Source Generator for Compile-Time Dispatch

[eneshoxha]

Title: Add source generator to eliminate reflection-based dispatch and enable NativeAOT

Labels: enhancement, mediator

Body:

Problem

Mediator.cs uses runtime reflection with ConcurrentDictionary<Type, MethodInfo> caches to dispatch commands, queries, and stream queries when the concrete type is only known at runtime (the single-generic-parameter overloads like SendCommandAsync<TResult>(ICommand<TResult>)). This has several drawbacks:

• Cold-start overhead from reflection and MakeGenericMethod on first call per type
• No NativeAOT / trimming support -- the reflection patterns are not trim-safe
• No compile-time validation -- a missing handler registration only fails at runtime
• Allocation pressure from new object[] { command, cancellationToken } on every reflected dispatch

Proposed Solution

Create a Cortex.Mediator.SourceGenerator package that uses Roslyn incremental source generators to:

1. Scan the compilation for all ICommand<T>, IQuery<T>, IStreamQuery<T>, and INotification implementations
2. Generate a GeneratedMediator class with strongly-typed dispatch (no reflection)
3. Generate DI registration code (AddCortexMediator()) that registers all handlers without Scrutor
4. Emit build warnings/errors for commands/queries without matching handlers
5. Support [assembly: MediatorAssembly] or similar attribute to control scanning scope

Benefits

• Zero reflection at runtime
• Full NativeAOT and trimming compatibility
• Missing handler = compile error
• Removes Scrutor dependency when using source gen
• Aligns with .NET ecosystem direction (similar approach to Mediator by martinothamar)

Scope

• New project: Cortex.Mediator.SourceGenerator
• The existing reflection-based Mediator class should remain as a fallback for dynamic scenarios
• Users opt in by adding the source generator package