.NET client: support trimming and Native AOT

Summary

Make the first-party .NET client introduced in #206 trimming-safe and compatible with Native AOT.

Current limitation

The current JSON path relies on runtime reflection:

• SystemTextJsonAhpSerializer uses reflection-based System.Text.Json serialization.
• UnionConverter<T> resolves and deserializes runtime variant types.
• WireEnumConverter<T> reflects over enum fields and WireValueAttribute metadata.
• Public serializer APIs are marked with RequiresUnreferencedCode and RequiresDynamicCode.
• The packages intentionally do not claim IsTrimmable or IsAotCompatible.

Proposed work

• Generate or maintain a JsonSerializerContext covering the complete generated protocol surface and hand-written envelope types.
• Update SystemTextJsonAhpSerializer to use generated JsonTypeInfo metadata.
• Replace reflection-based union and wire-enum lookup with generated/static metadata.
• Preserve unknown discriminants and wire values exactly as required by the protocol.
• Remove RequiresUnreferencedCode / RequiresDynamicCode from supported public paths.
• Set appropriate package compatibility properties (IsTrimmable / IsAotCompatible) once validated.
• Add CI coverage that publishes and runs a representative client with trimming and Native AOT enabled.

Acceptance criteria

• dotnet publish succeeds for a representative client with PublishAot=true and no AHP-originated trim/AOT warnings.
• Existing reducer and round-trip fixtures remain green.
• Unknown union discriminants continue to round-trip verbatim.
• Both netstandard2.0 and net8.0 package targets continue to build.

Start with SystemTextJsonAhpSerializer, UnionConverter, and WireEnumConverter; trace the generated protocol and hand-written envelope types they cover. Run the existing reducer and round-trip fixtures, then publish a representative client with PublishAot=true for the netstandard2.0 and net8.0 targets. Done means no AHP-originated trim/AOT warnings, unknown values round-trip, and both targets build.