v0.39.0

• New codegen targets for vgi-csharp: vgi.codegen.csharp_types (protocol records and enums as C# types) and vgi.codegen.csharp_schemas (the protocol's Arrow schemas, for vgi-csharp's conformance test). Both run from scripts/regen_generated.py; tests/test_generated_csharp.py catches drift.