feat(galaxy): writer borrows live subscription item handles (skip redundant AddItem)

GatewayGalaxyDataWriter now accepts an optional subscribedHandleSource
delegate; TryResolveCachedOrBorrowed checks _itemHandles first then the
source, so the first write to an already-subscribed tag skips the
AddItem round-trip. Borrowed handles are not cached (subscription
registry owns lifecycle). AddItemCallCount seam confirms gateway calls.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs

@@ -195,10 +195,10 @@ public sealed class GatewayGalaxyDataWriter : IGalaxyDataWriter
     private async Task<int> EnsureItemHandleAsync(
         MxGatewaySession session, int serverHandle, string fullRef, CancellationToken ct)
     {
-        if (_itemHandles.TryGetValue(fullRef, out var existing)) return existing;
+        if (TryResolveCachedOrBorrowed(fullRef) is int resolved) return resolved;
         var handle = await session.AddItemAsync(serverHandle, fullRef, ct).ConfigureAwait(false);
-        _itemHandles[fullRef] = handle;
         Interlocked.Increment(ref _addItemCallCount);
+        _itemHandles[fullRef] = handle;
         return handle;
     }