Proto3 definition with SiteStreamService (server streaming), Quality and
AlarmStateEnum enums with UNSPECIFIED=0, google.protobuf.Timestamp for
cross-platform timestamps. Pre-generated C# stubs checked in (no protoc
at build time). 10 roundtrip tests covering serialization, oneof
discrimination, and Timestamp<->DateTimeOffset conversion.
Define the SiteStreamService proto for real-time instance event
streaming (attribute value changes, alarm state changes) from site
nodes to central. Add pre-generated C# stubs following the existing
LmxProxy pattern, gRPC NuGet packages with FrameworkReference for
ASP.NET Core server types, and proto roundtrip tests.
