feat: add sitestream.proto definition and generated gRPC stubs

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.

tests/ScadaLink.Communication.Tests/Grpc/ProtoRoundtripTests.cs

@@ -126,4 +126,32 @@ public class ProtoRoundtripTests
         Assert.Equal(AlarmStateEnum.AlarmStateActive, deserialized.AlarmChanged.State);
         Assert.Equal(1, deserialized.AlarmChanged.Priority);
     }
+
+    [Fact]
+    public void Timestamp_DateTimeOffset_FullRoundTrip()
+    {
+        var original = new DateTimeOffset(2026, 3, 21, 14, 30, 45, 123, TimeSpan.Zero);
+        var update = new AttributeValueUpdate
+        {
+            InstanceUniqueName = "Motor-1",
+            AttributePath = "Speed",
+            AttributeName = "Speed",
+            Value = "42.5",
+            Quality = Quality.Good,
+            Timestamp = Timestamp.FromDateTimeOffset(original)
+        };
+
+        var bytes = update.ToByteArray();
+        var deserialized = AttributeValueUpdate.Parser.ParseFrom(bytes);
+
+        var roundTripped = deserialized.Timestamp.ToDateTimeOffset();
+        Assert.Equal(original.Year, roundTripped.Year);
+        Assert.Equal(original.Month, roundTripped.Month);
+        Assert.Equal(original.Day, roundTripped.Day);
+        Assert.Equal(original.Hour, roundTripped.Hour);
+        Assert.Equal(original.Minute, roundTripped.Minute);
+        Assert.Equal(original.Second, roundTripped.Second);
+        Assert.Equal(original.Millisecond, roundTripped.Millisecond);
+        Assert.Equal(TimeSpan.Zero, roundTripped.Offset);
+    }
 }