fix(drivers): #630 review - AbCip disposes alarm projection + clears tag tables, join not abortable by init token; S7 swaps Plc under _gate; doc rationale

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -226,7 +226,7 @@ public sealed class S7Driver
         // #630: InitializeAsync is reachable on a live driver (DriverInstanceActor.AdoptConfigDuringInit
         // runs it with no teardown first). Stop the previous probe loop and drop the previous connection
         // before they are overwritten below. Deliberately NOT ShutdownAsync: that also drops _poll and the
-        // subscriptions, which nothing would re-drive.
+        // subscriptions, which are owned by the shutdown path (the actor re-subscribes after every successful init).
         await ReleaseConnectionAsync().ConfigureAwait(false);
         Health = new DriverHealth(DriverState.Initializing, null, null);
         try
@@ -395,10 +395,22 @@ public sealed class S7Driver
         _probeCts = null;
         _probeTask = null;
 
-        _initialized = false;
-        _plcDead = false;
-        try { Plc?.Dispose(); } catch { /* best-effort — tearing down anyway */ }
-        Plc = null;
+        // Swap the connection out under _gate so a Read/Write/poll pass that already holds the gate (they
+        // re-read Plc inside it) can never see a disposed Plc mid-swap. Bounded: a wedged holder must not hang
+        // teardown, in which case we proceed exactly as the un-gated path always did.
+        var gated = false;
+        try { gated = await _gate.WaitAsync(DrainTimeout).ConfigureAwait(false); } catch { }
+        try
+        {
+            _initialized = false;
+            _plcDead = false;
+            try { Plc?.Dispose(); } catch { /* best-effort — tearing down anyway */ }
+            Plc = null;
+        }
+        finally
+        {
+            if (gated) { try { _gate.Release(); } catch (ObjectDisposedException) { } }
+        }
     }
 
     /// <inheritdoc />