feat: add sitestream.proto definition and generated gRPC stubs

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.

src/ScadaLink.Communication/Protos/sitestream.proto

@@ -0,0 +1,52 @@
+syntax = "proto3";
+option csharp_namespace = "ScadaLink.Communication.Grpc";
+package sitestream;
+
+import "google/protobuf/timestamp.proto";
+
+service SiteStreamService {
+  rpc SubscribeInstance(InstanceStreamRequest) returns (stream SiteStreamEvent);
+}
+
+message InstanceStreamRequest {
+  string correlation_id = 1;
+  string instance_unique_name = 2;
+}
+
+message SiteStreamEvent {
+  string correlation_id = 1;
+  oneof event {
+    AttributeValueUpdate attribute_changed = 2;
+    AlarmStateUpdate alarm_changed = 3;
+  }
+}
+
+enum Quality {
+  QUALITY_UNSPECIFIED = 0;
+  QUALITY_GOOD = 1;
+  QUALITY_UNCERTAIN = 2;
+  QUALITY_BAD = 3;
+}
+
+enum AlarmStateEnum {
+  ALARM_STATE_UNSPECIFIED = 0;
+  ALARM_STATE_NORMAL = 1;
+  ALARM_STATE_ACTIVE = 2;
+}
+
+message AttributeValueUpdate {
+  string instance_unique_name = 1;
+  string attribute_path = 2;
+  string attribute_name = 3;
+  string value = 4;
+  Quality quality = 5;
+  google.protobuf.Timestamp timestamp = 6;
+}
+
+message AlarmStateUpdate {
+  string instance_unique_name = 1;
+  string alarm_name = 2;
+  AlarmStateEnum state = 3;
+  int32 priority = 4;
+  google.protobuf.Timestamp timestamp = 5;
+}