Add Gateway Aggregation pattern

[JerrettDavis]

Goal

Add Gateway Aggregation as a first-class PatternKit pattern for composing several downstream calls behind a single API-facing operation.

Required scope

• Fluent runtime API for composing named downstream fetches into a response model.
• Source-generated factory path with diagnostics for invalid aggregation declarations.
• TinyBDD runtime, generator, and real-world example coverage.
• Importable IServiceCollection and ASP.NET Core endpoint example.
• User guide, generator guide, example docs, catalog coverage, and production readiness entries.
• CI must land green on main.

[JerrettDavis]

Reopened: PR #313 implemented Event Notification, not Gateway Aggregation. Gateway Aggregation remains to be implemented.