feat(mqtt): validate MQTTnet-5/net10 restore under central pinning (spike, P1 gate)

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

Directory.Packages.props

@@ -75,6 +75,15 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageVersion Include="Microsoft.Playwright" Version="1.51.0" />
     <PackageVersion Include="Moq" Version="4.20.72" />
+    <!--
+      MQTT client for the Mqtt / Sparkplug B driver. MQTTnet v5 is the first line shipping a
+      net10.0 TFM (lib/net10.0, and an EMPTY net10.0 dependency group — zero transitive
+      packages, so it cannot participate in a diamond under this repo's deliberately-OFF
+      CentralPackageTransitivePinningEnabled). Sparkplug B payloads are hand-rolled rather than
+      taken from SparkplugNet, whose newest release (1.3.10) still pins MQTTnet 4.3.6.1152 and
+      ships net6.0/net8.0 only.
+    -->
+    <PackageVersion Include="MQTTnet" Version="5.2.0.1603" />
     <PackageVersion Include="Novell.Directory.Ldap.NETStandard" Version="3.6.0" />
     <PackageVersion Include="OPCFoundation.NetStandard.Opc.Ua.Client" Version="1.5.378.106" />
     <PackageVersion Include="OPCFoundation.NetStandard.Opc.Ua.Configuration" Version="1.5.378.106" />

ZB.MOM.WW.OtOpcUa.slnx

@@ -42,6 +42,7 @@
     <Project Path="src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.csproj" />
     <Project Path="src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts.csproj" />
     <Project Path="src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Browser/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Browser.csproj" />
+    <Project Path="src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts.csproj" />
   </Folder>
   <Folder Name="/src/Drivers/Driver CLIs/">
     <Project Path="src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Cli.Common/ZB.MOM.WW.OtOpcUa.Driver.Cli.Common.csproj" />

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    </PropertyGroup>
+    <ItemGroup>
+        <!-- TEMPORARY (Task 0 spike): proves the MQTTnet-5 graph restores + builds on net10.0
+             under this repo's central package management. `.Contracts` is transport-free by
+             design — Task 1 removes this reference and it moves to the `.Driver` project. -->
+        <PackageReference Include="MQTTnet" />
+    </ItemGroup>
+</Project>