fix(ablegacy): coalesceReads absent means default; the form never pins it

Review r0 M9. The AdminUI form wrote coalesceReads:true on every save, pinning
today's default into every edited driver. AbLegacyDriverOptions.CoalesceReads
is now bool? (null = default ON, never serialised, same encoding as
Transport); the form omits the key when the switch is on and writes only an
explicit false. Persisted coalesceReads:true keeps loading as on.

Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Contracts/AbLegacyDriverOptions.cs

@@ -51,9 +51,12 @@ public sealed class AbLegacyDriverOptions
     /// <summary>
     ///     Whether scalar tags on adjacent elements of one N / B / F / L file are read as ONE multi-element PCCC read
     ///     (driver-config key <c>coalesceReads</c>, default <see langword="true"/>). A block the device refuses falls
-    ///     back to per-tag reads on its own; set this false to read every tag singly.
+    ///     back to per-tag reads on its own; set this false to read every tag singly. <see langword="null"/> (the key
+    ///     absent) means the default, ON - and null is never written, so a save through any serializer (the AdminUI
+    ///     form included) keeps a defaulted driver defaulted instead of pinning today's default (review 07 r0 M9).
     /// </summary>
-    public bool CoalesceReads { get; init; } = true;
+    [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull)]
+    public bool? CoalesceReads { get; init; }
 
     /// <summary>
     ///     Timeout for the AdminUI Test Connect probe, in seconds. The AdminUI clamps to a

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.Coalescing.cs

@@ -72,7 +72,7 @@ public sealed partial class AbLegacyDriver
     /// </summary>
     private List<ReadUnit> PlanReadUnits(DeviceState device, List<ReadItem> items)
     {
-        if (!_options.CoalesceReads || items.Count < 2) return [.. items.Select(i => new ReadUnit(i, null))];
+        if (_options.CoalesceReads == false || items.Count < 2) return [.. items.Select(i => new ReadUnit(i, null))];
 
         var units = new List<ReadUnit>(items.Count);
         var groups = new Dictionary<(string Letter, int File, AbLegacyDataType Type), List<ReadItem>>();

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverFactoryExtensions.cs

@@ -97,7 +97,7 @@ public static class AbLegacyDriverFactoryExtensions
                 ProbeAddress = dto.Probe?.ProbeAddress ?? "S:0",
             },
             Timeout = PositiveTimeoutOrDefault(dto.TimeoutMs, 2_000),
-            CoalesceReads = dto.CoalesceReads ?? true,
+            CoalesceReads = dto.CoalesceReads,
             // Absent ⇒ null ("use the default" = Libplctag, resolved by AbLegacyTransportResolver);
             // a bad value still fails exactly as any other bad enum field does.
             Transport = string.IsNullOrWhiteSpace(dto.Transport)

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

@@ -208,7 +208,7 @@
             _loadedProbeTimeout = o.Probe.Timeout,
             TimeoutSeconds = (int)o.Timeout.TotalSeconds,
             Transport = o.Transport,
-            CoalesceReads = o.CoalesceReads,
+            CoalesceReads = o.CoalesceReads ?? true,
             ProbeEnabled = o.Probe.Enabled,
             ProbeIntervalSeconds = (int)o.Probe.Interval.TotalSeconds,
             ProbeTimeoutSeconds = (int)o.Probe.Timeout.TotalSeconds,
@@ -229,7 +229,8 @@
             },
             Timeout = DriverConfigMs.Save(TimeoutSeconds, _loadedTimeout),
             Transport = Transport,
-            CoalesceReads = CoalesceReads,
+            // On = the default: OMIT the key (never pin a concrete default); off is the only value written.
+            CoalesceReads = CoalesceReads ? null : false,
             ProbeTimeoutSeconds = AdminProbeTimeoutSeconds,
         };
     }

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyReadCoalescingTests.cs

@@ -180,5 +180,5 @@ public sealed class AbLegacyReadCoalescingTests
     [InlineData("""{"coalesceReads":false,"devices":[{"hostAddress":"ab://10.0.0.5/1,0"}]}""", false)]
     [InlineData("""{"coalesceReads":true,"someFutureKey":1,"devices":[{"hostAddress":"ab://10.0.0.5/1,0"}]}""", true)]
     public void ParseOptions_reads_coalesceReads_defaulting_true(string json, bool expected) =>
-        AbLegacyDriverFactoryExtensions.ParseOptions("drv", json).CoalesceReads.ShouldBe(expected);
+        (AbLegacyDriverFactoryExtensions.ParseOptions("drv", json).CoalesceReads != false).ShouldBe(expected);
 }

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/AbLegacyDriverPageFormSerializationTests.cs

@@ -269,8 +269,12 @@ public sealed class AbLegacyDriverPageFormSerializationTests
 
         var saved = AbLegacyDriverForm.Serialize(form.ToOptions(loaded.Devices), original);
 
-        ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.AbLegacyDriverFactoryExtensions.ParseOptions("t", saved)
-            .CoalesceReads.ShouldBe(edited);
+        (ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.AbLegacyDriverFactoryExtensions.ParseOptions("t", saved)
+            .CoalesceReads != false).ShouldBe(edited);
+        // Review 07 r0 M9: ON is the default and is OMITTED, never pinned; only an explicit off is written.
+        var root = System.Text.Json.Nodes.JsonNode.Parse(saved)!.AsObject();
+        if (edited) root.ContainsKey("coalesceReads").ShouldBeFalse();
+        else root["coalesceReads"]!.GetValue<bool>().ShouldBeFalse();
     }
 
     [Fact]