Replace anonymous structs with named types

* Replace all anonymous nested structs with named types
* Eliminate RegisteredCountry duplication, use CountryRecord
* Optimize HasData methods with zero struct comparison
* Simplify parent HasData methods using substruct methods

This improves struct initialization ergonomics while maintaining
the same JSON serialization behavior and performance.

Author: oschwald

CHANGELOG.md

@@ -7,6 +7,11 @@
   from validation, allowing users to access network topology information even when
   no GeoIP data is found. The Network and IPAddress fields are now always
   populated for all lookups, regardless of whether GeoIP data is available.
+* **BREAKING CHANGE**: Replaced all anonymous nested structs with named types to
+  improve struct initialization ergonomics. All result structs (Enterprise, City,
+  Country) now use named types like `EnterpriseCityRecord`, `CityTraits`,
+  `CountryRecord`, etc. This makes it much easier to initialize structs in user
+  code while maintaining the same JSON serialization behavior.
 
 # 2.0.0-beta.1 - 2025-06-22