Add External Configuration Store cloud architecture pattern

[JerrettDavis]

Add a first-class External Configuration Store pattern to PatternKit.\n\nAcceptance criteria:\n- Fluent runtime API for typed configuration lookup, fallback, refresh, and validation\n- Source-generated path for typed configuration accessors\n- Production-style Microsoft.Extensions.Configuration and IServiceCollection example\n- Docs for runtime, generator, and example usage\n- TinyBDD runtime, generator, example, and catalog coverage\n- Pattern coverage catalog entry with no generator gap