fix(modbus): review r0 - FC23 opt-in, explicit maxReadGap 0 = off, legacy coil types load, one health writer

Coordinator rulings and Important findings from review r0:
1. FC23 is opt-in (new UseFC23ForMultiRegisterWrites, default false); a config
   without it - including the persisted "disableFC23": false every form-saved
   config carries - keeps the FC16 wire. DisableFC23 stays the hard kill-switch.
2. An explicit maxReadGap 0 means coalescing off again (its pre-lane meaning);
   the gap-8 default applies only to an absent key.
3. Coils/DiscreteInputs tags persisted with a register dataType (the old
   editor saved Int16) are coerced to Bool, never refused, with no deploy
   warning (a warning can be promoted to a blocking error). The editor loads
   them as Bool. Register region + Bool is still refused.
4. One comms-fault slot for reads and writes: a good read clears a write's
   transport fault and vice versa.
Minors: Dispose writes its own Unknown health (m1); persisted region/address
are authoritative over addressString so a Family change cannot remap a live
tag (m2); a malformed-but-delivered reply no longer fails the unit fast (m3);
a fresh socket published after a concurrent dispose is torn down (m4); the
editor resets address text per tag (m5); master merged (m6); the FC23 latch
log says re-init restores it (m7). Back-compat tests use the old persisted
JSON shapes and fail without the fixes.

Refs #741
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Drivers/Forms/ModbusDriverForm.razor

@@ -80,7 +80,7 @@
             <div class="col-md-3">
                 <label class="form-label">Max read gap (coalescing)</label>
                 <InputNumber @bind-Value="_form.MaxReadGap" @bind-Value:after="EmitAsync" class="form-control form-control-sm" />
-                <div class="form-text">Default 8. 0 = adjacent tags only. Refused ranges fall back per tag.</div>
+                <div class="form-text">0 = no coalescing. Default 8. Refused ranges fall back per tag.</div>
             </div>
             <div class="col-md-3">
                 <div class="form-check form-switch mt-4">
@@ -112,7 +112,13 @@
             <div class="col-md-4">
                 <div class="form-check form-switch">
                     <InputCheckbox @bind-Value="_form.DisableFC23" @bind-Value:after="EmitAsync" class="form-check-input" id="disableFC23" />
-                    <label class="form-check-label" for="disableFC23">Disable FC23 (multi-register writes use FC16 instead of FC23 write + read-back)</label>
+                    <label class="form-check-label" for="disableFC23">Disable FC23 (never send FC23, even when opted in below)</label>
+                </div>
+            </div>
+            <div class="col-md-4">
+                <div class="form-check form-switch">
+                    <InputCheckbox @bind-Value="_form.UseFC23ForMultiRegisterWrites" @bind-Value:after="EmitAsync" class="form-check-input" id="useFC23" />
+                    <label class="form-check-label" for="useFC23">Use FC23 write + read-back for multi-register writes (opt-in; default FC16)</label>
                 </div>
             </div>
             <div class="col-md-4">
@@ -324,6 +330,7 @@
         public bool UseFC15ForSingleCoilWrites { get; set; } = false;
         public bool UseFC16ForSingleRegisterWrites { get; set; } = false;
         public bool DisableFC23 { get; set; } = false;
+        public bool UseFC23ForMultiRegisterWrites { get; set; } = false;
         public bool WriteOnChangeOnly { get; set; } = false;
 
         // Keep-alive
@@ -368,6 +375,7 @@
             UseFC15ForSingleCoilWrites = o.UseFC15ForSingleCoilWrites,
             UseFC16ForSingleRegisterWrites = o.UseFC16ForSingleRegisterWrites,
             DisableFC23 = o.DisableFC23,
+            UseFC23ForMultiRegisterWrites = o.UseFC23ForMultiRegisterWrites,
             WriteOnChangeOnly = o.WriteOnChangeOnly,
             KeepAliveEnabled = o.KeepAlive.Enabled,
             KeepAliveTimeSeconds = (int)o.KeepAlive.Time.TotalSeconds,
@@ -404,6 +412,7 @@
             UseFC15ForSingleCoilWrites = UseFC15ForSingleCoilWrites,
             UseFC16ForSingleRegisterWrites = UseFC16ForSingleRegisterWrites,
             DisableFC23 = DisableFC23,
+            UseFC23ForMultiRegisterWrites = UseFC23ForMultiRegisterWrites,
             AutoProhibitReprobeInterval = AutoProhibitReprobeIntervalSeconds > 0
                 ? TimeSpan.FromSeconds(AutoProhibitReprobeIntervalSeconds) : null,
             MaxReadGap = (ushort)Math.Clamp(MaxReadGap, 0, 65535),

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/TagEditors/ModbusTagConfigEditor.razor

@@ -80,7 +80,10 @@
         if (ConfigJson == _lastConfigJson) { return; }
         _lastConfigJson = ConfigJson;
         _m = ModbusTagConfigModel.FromJson(ConfigJson);
-        _addressText ??= _m.AddressString;
+        // Reset on EVERY config change: a reused editor instance switched to another tag must not show the
+        // previous tag's address text.
+        _addressText = _m.AddressString;
+        _addressError = null;
         _family = ModbusTagConfigModel.ReadFamily(GetDriverConfigJson()).Family;
     }