Deployment/AOT checkpoint

Implement static string DeploymentDescription(bool selfContained, bool trimmed, bool aot): return "framework-dependent" when selfContained=false; "self-contained" when selfContained && !trimmed && !aot; "trimmed" when selfContained && trimmed && !aot; "aot" when aot (regardless of trimmed — AOT implies it). Then implement static bool IsTrimCompatible(string reflectionPattern) returning false for patterns the linker cannot prove: "assembly scanning", "activator by name", "json by string type name"; true for "source generated serializer", "generic constraint", "explicit registration".