fix(galaxy): authoritative handle resolution + review cleanups

Make SubscriptionRegistry.TryResolveItemHandle confirm a live subscription
genuinely binds fullRef->handle (via the reverse index) rather than trusting
the forward-map hint + a bare liveness check. Fixes the cross-ref-same-handle
hazard (wrong-tag borrow) while preserving the legitimate
multiple-subscriptions-per-tag borrow. Adds cross-ref + same-ref-multi-sub
tests; drops a duplicate SubscriptionEntry <summary>; documents the writer's
supervisory-advise reconnect lifecycle.

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

@@ -210,7 +210,9 @@ public sealed class GatewayGalaxyDataWriter : IGalaxyDataWriter
     ///     a regular Advise instead; we don't log in, so we use the supervisory context.)
     ///     The gateway client doesn't expose a typed method, so we build the <see cref="MxCommand"/>
     ///     and route through <c>InvokeAsync</c> (same pattern as <see cref="InvokeWriteSecuredAsync"/>).
-    ///     Idempotent per handle for the session lifetime.
+    ///     Idempotent per handle for the session lifetime. The keyed-by-handle dedupe is reset on a
+    ///     session reconnect via <see cref="InvalidateHandleCaches"/>, so a fresh session always
+    ///     re-advises — the supervisory state never outlives the handle it was taken against.
     /// </summary>
     private async Task EnsureSupervisoryAdvisedAsync(
         MxGatewaySession session, int serverHandle, int itemHandle, CancellationToken ct)

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

@@ -124,18 +124,31 @@ internal sealed class SubscriptionRegistry
     /// <summary>
     ///     Resolve the live MXAccess item handle a current subscription holds for <paramref name="fullRef"/>,
     ///     or null when no live subscription covers it. The Galaxy writer borrows this handle to skip a
-    ///     redundant AddItem round-trip on the first write to an already-subscribed tag. Guarded by the
+    ///     redundant AddItem round-trip on the first write to an already-subscribed tag.
-    ///     authoritative live-handle set (<c>_subscribersByItemHandle</c>) so a stale forward-map entry
-    ///     can never hand out a dead handle.
     /// </summary>
+    /// <remarks>
+    ///     The <c>_itemHandleByFullRef</c> forward map is only a hint — resolution is authoritative:
+    ///     it confirms a <em>live</em> subscription actually binds this <paramref name="fullRef"/> to
+    ///     the candidate handle (via the per-entry reverse index), so a stale forward entry can never
+    ///     hand the writer a dead handle, nor a handle that now belongs to a different tag. This keeps
+    ///     a tag resolvable while ANY subscription still binds it (the legitimate
+    ///     multiple-driver-subscriptions-per-tag case) yet returns null the moment the last one drops.
+    /// </remarks>
     /// <param name="fullRef">The dotted tag full reference (e.g. <c>TestMachine_002.TestFloat</c>).</param>
     /// <returns>The live item handle, or null when none is currently subscribed.</returns>
     public int? TryResolveItemHandle(string fullRef)
     {
         if (fullRef is null) return null;
-        if (_itemHandleByFullRef.TryGetValue(fullRef, out var handle)
+        if (!_itemHandleByFullRef.TryGetValue(fullRef, out var handle)) return null;
-            && _subscribersByItemHandle.ContainsKey(handle))
+        if (!_subscribersByItemHandle.TryGetValue(handle, out var subs)) return null; // dead handle
-            return handle;
+
+        // Confirm a live subscription genuinely binds fullRef -> handle (not a stale forward entry
+        // pointing at a handle the gateway has since associated with another tag).
+        foreach (var subId in subs)
+            if (_bySubscriptionId.TryGetValue(subId, out var entry)
+                && entry.FullRefByItemHandle.TryGetValue(handle, out var liveRef)
+                && string.Equals(liveRef, fullRef, StringComparison.OrdinalIgnoreCase))
+                return handle;
         return null;
     }
 
@@ -201,7 +214,6 @@ internal sealed class SubscriptionRegistry
     ///     (Driver.Galaxy-012). Failed bindings (item handle ≤ 0) are excluded from the
     ///     index because the EventPump only dispatches for positive handles.
     /// </summary>
-    /// <summary>Per-subscription bookkeeping entry.</summary>
     private sealed class SubscriptionEntry
     {
         /// <summary>Gets the subscription identifier.</summary>

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/SubscriptionRegistryHandleResolveTests.cs

@@ -114,4 +114,43 @@ public sealed class SubscriptionRegistryHandleResolveTests
         // must return null regardless of whether the forward entry was cleaned up.
         registry.TryResolveItemHandle("Tag.A").ShouldBeNull();
     }
+
+    /// <summary>
+    ///     A tag may legitimately appear in multiple driver subscriptions (separate OPC UA monitored
+    ///     items on the same Galaxy attribute) — they share one gw item handle. Removing ONE of them
+    ///     must keep the tag resolvable while another subscription still binds it, so the writer keeps
+    ///     borrowing instead of falling back to AddItem.
+    /// </summary>
+    [Fact]
+    public void Remove_OneOfTwoSubscribersForSameRef_StillResolves()
+    {
+        var registry = new SubscriptionRegistry();
+        registry.Register(1, [new TagBinding("Tag.A", 5)]);
+        registry.Register(2, [new TagBinding("Tag.A", 5)]);
+
+        registry.Remove(1);
+
+        // sub2 still binds Tag.A -> 5, so the borrow must still be offered.
+        registry.TryResolveItemHandle("Tag.A").ShouldBe(5);
+    }
+
+    /// <summary>
+    ///     Authoritative resolution: if two DIFFERENT refs ever map to the same numeric handle and the
+    ///     subscription for one is removed, that ref must NOT resolve to the handle that now belongs to
+    ///     the other ref — a wrong-tag write would be the worst outcome. Resolution confirms a live
+    ///     subscription genuinely binds fullRef -> handle, not just that the handle is alive.
+    /// </summary>
+    [Fact]
+    public void Remove_CrossRefSameHandle_DoesNotResolveToTheOtherRefsHandle()
+    {
+        var registry = new SubscriptionRegistry();
+        registry.Register(1, [new TagBinding("Tag.A", 5)]);
+        registry.Register(2, [new TagBinding("Tag.B", 5)]);
+
+        registry.Remove(1);
+
+        // Tag.A's subscription is gone; handle 5 is still alive but now only Tag.B binds it.
+        registry.TryResolveItemHandle("Tag.A").ShouldBeNull();
+        registry.TryResolveItemHandle("Tag.B").ShouldBe(5);
+    }
 }