docs(design): VirtualTag/script memory scalability (A0+A+guardrail; C2 deferred) + measurement harness

tools/mem-probe/LeanContext/LeanContext.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <!-- Throwaway memory probe: keep build noise low. -->
+    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
+    <GenerateDocumentationFile>false</GenerateDocumentationFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- Closure of THIS assembly = {LeanContext, Core.Abstractions}. No Roslyn. -->
+    <ProjectReference Include="..\..\..\src\Core\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj" />
+  </ItemGroup>
+
+</Project>

tools/mem-probe/LeanContext/LeanCtx.cs

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+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace LeanContext;
+
+/// <summary>
+///     LEAN globals type for the memory probe. Its transitive reference closure is only
+///     {LeanContext, Core.Abstractions} — deliberately NO Roslyn — so the per-script cost
+///     of the Roslyn reference-manager loading the globalsType's closure (dotnet/roslyn#22219)
+///     can be measured against the heavy <c>VirtualTagContext</c>, whose closure pulls in
+///     Microsoft.CodeAnalysis.CSharp.Scripting.
+///     <para>
+///         <see cref="GetTag"/> returns the same <see cref="DataValueSnapshot"/> type that
+///         <c>VirtualTagContext.GetTag</c> returns, so the probe's script source
+///         (<c>ctx.GetTag("x").Value</c>) is byte-identical for both modes — the ONLY
+///         difference is which assembly closure the globalsType lives in.
+///     </para>
+/// </summary>
+public sealed class LeanCtx
+{
+    private readonly System.Collections.Generic.Dictionary<string, DataValueSnapshot> _d = new();
+
+    public DataValueSnapshot GetTag(string p) =>
+        _d.TryGetValue(p, out var v) ? v : new DataValueSnapshot(null, 0u, null, default);
+}
+
+/// <summary>
+///     LEAN analogue of the prod <c>ScriptGlobals&lt;TContext&gt;</c> wrapper: exposes a
+///     named <c>ctx</c> property so the script source can be byte-identical to the heavy
+///     path (<c>ctx.GetTag(...).Value</c>). Lives in the LeanContext assembly, so its
+///     reference closure is {LeanContext, Core.Abstractions} — NO Roslyn. This is the A0
+///     "globals type in a lean assembly" treatment.
+/// </summary>
+public sealed class LeanGlobals
+{
+    public LeanCtx ctx { get; set; } = new();
+}