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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusDriverOptions.cs

@@ -107,14 +107,25 @@ public sealed class ModbusDriverOptions
     public TimeSpan? AutoProhibitReprobeInterval { get; init; } = null;
 
     /// <summary>
-    ///     Block-read coalescing budget. When non-zero, the read planner combines tags
-    ///     in the same (UnitId, Region) group whose addresses are at most this many registers
-    ///     apart into a single FC03/FC04/FC01/FC02 read. The sliced response is then dispatched
-    ///     back to per-tag values. Default <c>0</c> = no coalescing — every tag gets its own
-    ///     PDU (preserves the previous behaviour). Typical opt-in values are 5..32 — large enough
-    ///     to bridge a few unused registers, small enough to avoid trampling protected holes.
+    ///     Block-read coalescing budget. The read planner combines tags in the same (UnitId, Region)
+    ///     group whose addresses are at most this many registers (Coils / DiscreteInputs: bits) apart
+    ///     into a single FC03/FC04/FC01/FC02 read, then slices the response back into per-tag values.
+    ///     <c>0</c> coalesces only adjacent or overlapping tags (several <c>BitInRegister</c> tags on one
+    ///     status word still fold into one read). Default <c>8</c>. A range the device refuses is
+    ///     auto-prohibited and read per tag, so a protected hole costs one extra round trip, not data. To
+    ///     turn coalescing off entirely set <see cref="DisableReadCoalescing"/>, or exempt one tag with
+    ///     its <c>coalesceProhibited</c> key.
     /// </summary>
-    public ushort MaxReadGap { get; init; } = 0;
+    public ushort MaxReadGap { get; init; } = DefaultMaxReadGap;
+
+    /// <summary>The default <see cref="MaxReadGap"/> (shared by the factory DTO and the AdminUI form).</summary>
+    public const ushort DefaultMaxReadGap = 8;
+
+    /// <summary>
+    ///     When <c>true</c>, every tag is read with its own PDU (no block coalescing at all) — for a device
+    ///     that mis-answers merged reads in a way the auto-prohibition cannot catch. Default <c>false</c>.
+    /// </summary>
+    public bool DisableReadCoalescing { get; init; } = false;
 
     /// <summary>
     ///     PLC family hint that drives the parser's family-native branch. When set to a