Add Gateway Routing pattern

[JerrettDavis]

Goal

Add Gateway Routing as a first-class PatternKit pattern for dispatching API requests to named downstream capabilities from a gateway boundary.

Required scope

• Fluent runtime API for route predicates, route names, and typed handlers.
• Source-generated factory path with diagnostics for invalid route 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.