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.Commons/Observability/ScadaBridgeTelemetry.cs

@@ -93,6 +93,20 @@ public static class ScadaBridgeTelemetry
         Meter.CreateCounter<long>("scadabridge.site.stream.events_dropped", unit: "1",
             description: "Events evicted from a site gRPC stream's bounded send channel, tagged by stream kind.");
 
+    // ---------------- Histograms ----------------
+
+    /// <summary>
+    /// Distribution of how many events each site→central stream frame carried (R2 — gRPC
+    /// event batching), tagged by stream kind. Recorded ONLY for subscriptions that
+    /// negotiated batching, so the series' very existence says "this central speaks the
+    /// batched wire". A distribution pinned at 1 means the coalescing window never sees a
+    /// backlog (the site is quiet, or the window is too small to be earning anything);
+    /// mass at the size cap means the cap, not the window, is the binding constraint.
+    /// </summary>
+    private static readonly Histogram<int> _siteStreamBatchSize =
+        Meter.CreateHistogram<int>("scadabridge.site.stream.batch_size", unit: "1",
+            description: "Events per site gRPC stream frame (1 = unbatched frame), tagged by stream kind.");
+
     // ---------------- Observable gauges ----------------
 
     /// <summary>Current count of open site connections, mutated via <see cref="Interlocked"/>.</summary>
@@ -178,6 +192,16 @@ public static class ScadaBridgeTelemetry
     public static void RecordSiteStreamEventDropped(string streamKind) =>
         _siteStreamEventDrops.Add(1, new KeyValuePair<string, object?>("stream", streamKind));
 
+    /// <summary>
+    /// Records how many events one site gRPC stream frame carried. Called once per emitted
+    /// frame on a batching-negotiated subscription (never on an un-negotiated one, where it
+    /// would degenerate into a per-event instrument on the hottest path in the product).
+    /// </summary>
+    /// <param name="streamKind">Stream kind tag (<c>instance</c> or <c>site-alarms</c>).</param>
+    /// <param name="events">Events packed into the frame; 1 for a plain unbatched frame.</param>
+    public static void RecordSiteStreamBatchSize(string streamKind, int events) =>
+        _siteStreamBatchSize.Record(events, new KeyValuePair<string, object?>("stream", streamKind));
+
     /// <summary>
     /// Registers the provider the StoreAndForward queue-depth gauge reads on each observation.
     /// A later task supplies a provider that reads the real StoreAndForward depth. A null