Add Backends for Frontends pattern

[JerrettDavis]

Goal

Add Backends for Frontends as a first-class PatternKit pattern for shaping client-specific service facades over shared backend capabilities.

Required scope

• Fluent runtime API for named client experiences and typed handler composition.
• Source-generated factory path with diagnostics for invalid frontend 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.