- Add externalSchemas option for sideloading schemas during import resolution
- Add allowImport option to enable / processing
- Process chained imports (validation -> extended -> core metaschemas)
- Handle nested namespace objects in definitions (e.g., features namespace)
- Add meta submodule pointing to json-structure/meta repository
- Add meta schema tests to Python, TypeScript, Go, .NET, and Java SDKs
- Update README documentation for all SDKs with externalSchemas examples

SDK-specific changes:
- Python: Add _process_imports_in_external_schema method
- TypeScript: Add validateDefinitionOrNamespace, import processing in constructor
- Go: Add ExternalSchemas field, processImports method
- .NET: Add ExternalSchemas to ValidationResult, fix CollectDefinedRefsRecursive
- Java: Add ValidationOptions fields, import processing methods