feat(modbus): block-read coalescing on by default; coils/DI/BitInRegister/String fold

U2: MaxReadGap defaults to 8 and 0 now means adjacent-only rather than
off; a new DisableReadCoalescing switch (factory DTO + AdminUI form) turns it
off entirely. The planner also merges Coils/DiscreteInputs (FC01/FC02, gap in
bits), String scalars and BitInRegister tags - sixteen bit tags on one status
word are one FC03 instead of sixteen. Auto-prohibition and its re-probe /
bisection use the region's own function code (a coil range was re-probed
with FC04).

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 +80,13 @@
             <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">0 = no coalescing. Typical 5–32.</div>
+                <div class="form-text">Default 8. 0 = adjacent tags only. Refused ranges fall back per tag.</div>
+            </div>
+            <div class="col-md-3">
+                <div class="form-check form-switch mt-4">
+                    <InputCheckbox @bind-Value="_form.DisableReadCoalescing" @bind-Value:after="EmitAsync" class="form-check-input" id="disableCoalescing" />
+                    <label class="form-check-label" for="disableCoalescing">Disable read coalescing</label>
+                </div>
             </div>
         </div>
     </div>
@@ -311,7 +317,8 @@
         public int MaxRegistersPerRead { get; set; } = 125;
         public int MaxRegistersPerWrite { get; set; } = 123;
         public int MaxCoilsPerRead { get; set; } = 2000;
-        public int MaxReadGap { get; set; } = 0;
+        public int MaxReadGap { get; set; } = ModbusDriverOptions.DefaultMaxReadGap;
+        public bool DisableReadCoalescing { get; set; } = false;
 
         // Write options
         public bool UseFC15ForSingleCoilWrites { get; set; } = false;
@@ -357,6 +364,7 @@
             MaxRegistersPerWrite = o.MaxRegistersPerWrite,
             MaxCoilsPerRead = o.MaxCoilsPerRead,
             MaxReadGap = o.MaxReadGap,
+            DisableReadCoalescing = o.DisableReadCoalescing,
             UseFC15ForSingleCoilWrites = o.UseFC15ForSingleCoilWrites,
             UseFC16ForSingleRegisterWrites = o.UseFC16ForSingleRegisterWrites,
             DisableFC23 = o.DisableFC23,
@@ -399,6 +407,7 @@
             AutoProhibitReprobeInterval = AutoProhibitReprobeIntervalSeconds > 0
                 ? TimeSpan.FromSeconds(AutoProhibitReprobeIntervalSeconds) : null,
             MaxReadGap = (ushort)Math.Clamp(MaxReadGap, 0, 65535),
+            DisableReadCoalescing = DisableReadCoalescing,
             Family = Family,
             MelsecSubFamily = MelsecSubFamily,
             Transport = Transport,