Add Event-Driven Consumer enterprise integration pattern

[JerrettDavis]

Add a production-ready Event-Driven Consumer pattern slice.

Required scope:

• Fluent runtime API in PatternKit.Core for subscribing handlers to incoming channel events.
• Source-generated factory path and generator attributes.
• TinyBDD runtime, generator, attribute, example, and catalog coverage.
• Real-world IServiceCollection example that can be imported through AddPatternKitExamples().
• Runtime, generator, example, and user-guide docs.
• Hosted CI, docs, and CodeQL green before merge.