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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ScriptedTransport.cs

@@ -30,6 +30,13 @@ internal sealed class ScriptedTransport : IModbusTransport
 
     public int TotalSends => _sendsByUnit.Values.Sum();
 
+    /// <summary>Every PDU sent, in order.</summary>
+    public ConcurrentQueue<(byte Unit, byte[] Pdu)> Sent { get; } = new();
+
+    /// <summary>The (function code, start, quantity) of every sent request with that function code.</summary>
+    public List<(byte Fc, ushort Start, ushort Qty)> Requests(byte fc) =>
+        [.. Sent.Where(s => s.Pdu[0] == fc).Select(s => (s.Pdu[0], (ushort)((s.Pdu[1] << 8) | s.Pdu[2]), (ushort)((s.Pdu[3] << 8) | s.Pdu[4])))];
+
     public async Task ConnectAsync(CancellationToken ct)
     {
         ConnectEntered.TrySetResult();
@@ -39,6 +46,7 @@ internal sealed class ScriptedTransport : IModbusTransport
     public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
     {
         _sendsByUnit.AddOrUpdate(unitId, 1, (_, n) => n + 1);
+        Sent.Enqueue((unitId, pdu));
         var scripted = Handler?.Invoke(unitId, pdu, ct);
         return scripted ?? Bank.SendAsync(unitId, pdu, ct);
     }