feat(comm): batch site→central stream events over gRPC (R2)

Every AttributeValueChanged/AlarmStateChanged rode its own gRPC message on
SiteStreamService; at target scale that is ~37.5k messages/s/site of pure
framing overhead. Coalesce them, additively, with no new RPC.

Wire (sitestream.proto, regenerated via docker/regen-proto.sh sitestream):
  InstanceStreamRequest.batching_supported = 3
  SiteStreamRequest.batching_supported     = 2
  SiteStreamEvent.batch                    = 4  (new oneof case)
  SiteStreamEventBatch { repeated SiteStreamEvent events = 1 }

The proto3 default of batching_supported IS the negotiation, and it is
load-bearing: a batch frame reaches a pre-R2 central as EventOneofCase.None,
whose ConvertToDomainEvent returns null — the whole batch would vanish with
no error anywhere. An old central cannot set the flag so it never receives
one; an old site ignores the unknown request field and keeps sending
per-event frames, which the new client's ForEachEvent handles as the
single-event case. Both skew directions are covered by tests that go through
a real proto serialize/parse round-trip.

Server: SiteStreamEventBatcher, a per-subscriber pump replacing the handler's
await-foreach/WriteAsync loop and byte-identical to it at a cap of 1. It
never delays a lone event — it drains the already-queued backlog for free and
lingers only once a backlog is proven — emits a single-event buffer as a
plain frame, preserves order and per-event Timestamps exactly, and flushes
what is buffered when the send channel's writer completes. It sits DOWNSTREAM
of StreamRelayActor's bounded DropOldest channel, so it changes framing only
and does not move the burst ceiling (deferred register row 31, which lives in
the shared publish stage upstream of the BroadcastHub).

Client: sets the flag on both subscriptions and unpacks in order into the
existing per-event pipeline, so SiteAlarmAggregatorActor,
DebugStreamBridgeActor, consumer-keepalive/orphan logic,
reconnect-on-graceful-completion, generation fencing, the (siteId, endpoint)
factory key and IsLive semantics are untouched.

Options (validated): GrpcStreamBatchMaxEvents 100 (1 disables),
GrpcStreamBatchWindow 25 ms — validated strictly under 250 ms, the load
test's end-to-end P99 threshold. Measured worst case (trickle-with-backlog,
window-bound rather than cap-bound): P50 13.8 ms, P99 25.4 ms, max 25.8 ms;
cap-bound case sent 600 queued events in 6 frames.

Telemetry: histogram scadabridge.site.stream.batch_size tagged by stream
kind, recorded only on negotiated streams (per-event otherwise). It rides
ScadaBridgeTelemetry.MeterName, already in the ObservedMeters allowlist.

Docs: Component-Communication.md gains an Event Batching section; CLAUDE.md's
gRPC streaming bullet records the wire shape and the negotiation rationale.

src/ZB.MOM.WW.ScadaBridge.Communication/Protos/sitestream.proto

@@ -20,6 +20,14 @@ service SiteStreamService {
 message InstanceStreamRequest {
   string correlation_id = 1;
   string instance_unique_name = 2;
+  // Client-declared BATCH NEGOTIATION (R2, event batching). When true the client
+  // understands the SiteStreamEventBatch oneof case and the server may coalesce
+  // consecutive events into one frame. proto3 defaults this to false, so an OLD
+  // central that never sets it keeps receiving one frame per event — that default
+  // IS the negotiation, and it is what makes new-site↔old-central safe. A NEW
+  // central sets it against an OLD site, which ignores the unknown field and
+  // keeps sending per-event frames the new client also accepts. Additive-only.
+  bool batching_supported = 3;
 }
 
 // Request for the site-wide, alarm-only SubscribeSite stream. Unlike
@@ -27,6 +35,8 @@ message InstanceStreamRequest {
 // transitions for every instance on the site.
 message SiteStreamRequest {
   string correlation_id = 1;
+  // See InstanceStreamRequest.batching_supported. Additive-only.
+  bool batching_supported = 2;
 }
 
 message SiteStreamEvent {
@@ -34,9 +44,26 @@ message SiteStreamEvent {
   oneof event {
     AttributeValueUpdate attribute_changed = 2;
     AlarmStateUpdate alarm_changed = 3;
+    // Coalesced frame (R2). Emitted ONLY when the subscription request set
+    // batching_supported = true. A batch is never nested inside a batch, and a
+    // single event is always sent as a plain attribute_changed/alarm_changed
+    // frame — so a quiet stream's wire shape is byte-identical to before.
+    SiteStreamEventBatch batch = 4;
   }
 }
 
+// Coalesced carrier for several consecutive stream events (R2). Ordering is
+// significant: events appear in the exact order the site produced them, and the
+// client unpacks them in order into the same per-event pipeline, so per-event
+// Timestamp fidelity and downstream sequencing are unchanged.
+//
+// The inner events deliberately leave correlation_id EMPTY — the enclosing
+// SiteStreamEvent carries it once for the whole frame, which is the byte saving
+// batching exists for. No consumer reads the inner correlation_id.
+message SiteStreamEventBatch {
+  repeated SiteStreamEvent events = 1;
+}
+
 enum Quality {
   QUALITY_UNSPECIFIED = 0;
   QUALITY_GOOD = 1;