test(loadharness)+docs: wire-path stream probes — R2 batching benchmark-proven at target scale

The WP-4 runs measured the stream path only up to the bounded send channel;
the probe deliberately substituted the socket hop with a controllable reader,
which is exactly where the R2 SiteStreamEventBatcher lives — so batching had
never been in any benchmark's hot path. The harness now also attaches per-site
wire probes: a real Kestrel h2c listener hosting the production
SiteStreamGrpcServer, dialed by the production SiteStreamGrpcClient with
batching negotiated, batch sizes read from the production batch_size histogram
via a MeterListener (--wire-probes-per-site, default 10; 0 restores the old
shape). Full-scale run (10x500x75, 20-min window, results doc §8 + raw JSON):
899,807 events in 12,462 frames (mean 72.2 ev/frame, p50=p99=75), wire P99
43.4 ms vs the 250 ms budget — every batch pays the full 25 ms linger since a
75-event burst never hits the 100 cap — zero drops, zero faults, offered load
100.0% of nominal with fan-out doubled. CI smoke Fact now asserts the wire
path carries traffic and observes frames.

tests/ZB.MOM.WW.ScadaBridge.LoadHarness/ZB.MOM.WW.ScadaBridge.LoadHarness.csproj

@@ -27,6 +27,14 @@
     <ConcurrentGarbageCollection>true</ConcurrentGarbageCollection>
   </PropertyGroup>
 
+  <ItemGroup>
+    <!-- Wire-path probes: each site hosts the production SiteStreamGrpcServer on a
+         real Kestrel h2c listener so the R2 batcher + serialization + HTTP/2 flow
+         control sit inside the measured path (see Probes/SiteWireStreamHost.cs). -->
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+    <PackageReference Include="Grpc.AspNetCore" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.Commons/ZB.MOM.WW.ScadaBridge.Commons.csproj" />
     <ProjectReference Include="../../src/ZB.MOM.WW.ScadaBridge.SiteRuntime/ZB.MOM.WW.ScadaBridge.SiteRuntime.csproj" />