feat(mesh): FetchAndCache apply path — fetch->cache->apply-from-bytes, null=apply-failure

Phase 3 Task 5. Under ConfigSource:Mode=FetchAndCache, a DispatchDeployment whose
revision differs from _currentRevision kicks off a gRPC artifact fetch that PipeTo's
its result back to Self as FetchedForApply (never a blocking await in a receive — that
would freeze the mailbox for the fetch deadline). HandleFetchedForApply then applies
synchronously on the actor thread, mirroring ApplyAndAck: null bytes = apply FAILURE
(keep last-known-good, do not advance the revision, ack Failed — #485); non-null bytes
reconcile drivers, rebuild the address space and push subscriptions FROM the bytes in
hand (OpcUaPublishActor never reads central SQL), then cache them. A faulted fetch task
maps to null bytes so it can't escape as an unhandled message.

Extracted ReconcileDriversFromBlob from ReconcileDrivers so Direct-read, FetchAndCache
and boot-from-cache share one reconcile body (the #485 empty guard moves into it);
ApplyCachedArtifact now reuses it instead of duplicating the spawn-plan logic.

DI: DriverHostActor.Props gains fetchAndCacheMode + artifactFetcher (LAST, per the
positional-forwarding warning); Runtime resolves them from IOptions<ConfigSourceOptions>
(fetcher only in FetchAndCache mode); Host registers GrpcDeploymentArtifactFetcher under
hasDriver.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -30,6 +30,7 @@ using ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway;
 using ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Recorder;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.OpcUaServer;
+using ZB.MOM.WW.OtOpcUa.Runtime.DeploymentCache;
 using ZB.MOM.WW.OtOpcUa.Runtime.Historian;
 using ZB.MOM.WW.OtOpcUa.Runtime.Scripting;
 using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;
@@ -186,6 +187,12 @@ if (hasDriver)
     // (initiator) / LocalDb:SyncListenPort (listener) are set. See LocalDbRegistration.
     builder.Services.AddOtOpcUaLocalDb(builder.Configuration);
 
+    // Node-side gRPC artifact fetcher (per-cluster mesh Phase 3). Resolved + used by DriverHostActor
+    // only under ConfigSource:Mode = FetchAndCache; idle (never invoked) in the default Direct mode, so
+    // registering it unconditionally on driver nodes is harmless. IDisposable — DI disposes its cached
+    // h2c channels at shutdown.
+    builder.Services.AddSingleton<IDeploymentArtifactFetcher, GrpcDeploymentArtifactFetcher>();
+
     // gRPC server plumbing (AddGrpc + the two path-scoped auth interceptors) is registered once,
     // below, for hasDriver || hasAdmin — the LocalDb passive sync endpoint (driver) and the
     // ConfigServe artifact endpoint (admin) share one pipeline.