feat(mqtt): bespoke passive #-observation browser (plain)

MQTT has no discovery protocol, so the AdminUI address picker learns the topic
space the only way there is: subscribe to the wildcard and accumulate what
arrives. MqttBrowseSession serves that observation as a segment tree (split on
'/'); MqttDriverBrowser opens it with a distinct "{clientId}-browse-{guid8}"
identity under a 5-30 s clamped budget.

Browsing publishes NOTHING - the load-bearing safety property, since the picker
runs against a live plant broker. Every outbound message must route through the
single PublishAsync seam that counts into PublishCountForTest; P2's operator-
triggered Sparkplug rebirth is the one intended exception. Sparkplug mode fails
fast rather than serving a raw spBv1.0 topic tree that looks like a metric tree.

Deviation from the plan's ref list: the project also references .Driver so the
browse CONNECT reuses MqttConnection.BuildClientOptions (TLS posture, CA-pin
chain validator, credentials). A second copy would be a security divergence.

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

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

@@ -0,0 +1,36 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <LangVersion>latest</LangVersion>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Browser</RootNamespace>
+        <AssemblyName>ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Browser</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Commons\ZB.MOM.WW.OtOpcUa.Commons.csproj"/>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts\ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts.csproj"/>
+        <!--
+            Deviation from the plan's ref list (which named .Contracts only): the browse connect
+            reuses MqttConnection.BuildClientOptions rather than reimplementing it. That method
+            owns the TLS posture, the CA-pin chain validator and the credential handling; a second
+            copy here would be a security divergence waiting to happen (browse silently not
+            honouring a pinned CA while the runtime driver does). It is pure — no I/O, no network —
+            so the dependency costs nothing at browse time.
+        -->
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Mqtt\ZB.MOM.WW.OtOpcUa.Driver.Mqtt.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="MQTTnet"/>
+        <PackageReference Include="Microsoft.Extensions.Logging.Abstractions"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests"/>
+    </ItemGroup>
+
+</Project>