feat(host): F8b RoslynVirtualTagEvaluator — production virtual-tag eval

RoslynVirtualTagEvaluator wraps Core.Scripting.ScriptEvaluator + Core
.VirtualTags.VirtualTagContext into a single-tag IVirtualTagEvaluator
adapter. Caches the compiled ScriptEvaluator per unique expression so
the second-and-onwards Evaluate is an in-process method call against the
dependency dictionary. Compile/sandbox/runtime errors all surface as
VirtualTagEvalResult.Failure rather than propagating exceptions through
the VirtualTagActor message loop.

Single-tag scope: cross-tag ctx.SetVirtualTag writes are dropped + logged
because fan-out between actors is owned by DependencyMuxActor. Cycle
detection + cascade ordering stay in Core.VirtualTags.VirtualTagEngine
where they belong (loaded fleet-wide); this adapter keeps the actor
message handler simple.

Host adds Core.Scripting + Core.VirtualTags project refs, plus a
TargetWarningsAsErrors NU1608 suppression — Microsoft.CodeAnalysis.CSharp
.Scripting 4.12.0 pins Common to 4.12.0 but ASP.NET Core transitively
brings Microsoft.CodeAnalysis.Common 5.0.0; the surface we use is stable
across the drift (verified by Core.Scripting.Tests).

Program.cs binds RoslynVirtualTagEvaluator → IVirtualTagEvaluator on
driver-role hosts, replacing the F8-default NullVirtualTagEvaluator so
VirtualTagActor evaluates real user scripts at runtime.

6 new adapter tests cover: simple expression sums, cache reuse across
calls, compile-error denial, runtime-throw denial, empty-expression
denial, post-dispose denial. Host.IntegrationTests now 10/10 green.

Closes #79. F9b + #107 next.

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/RoslynVirtualTagEvaluatorTests.cs

@@ -0,0 +1,92 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Host.Engines;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+/// F8b — verifies <see cref="RoslynVirtualTagEvaluator"/> compiles user expressions through
+/// the Core.Scripting sandbox, runs them against the dependency dictionary, caches the
+/// compiled assembly per source, and surfaces failures (compile error, sandbox violation,
+/// runtime throw) as <c>VirtualTagEvalResult.Failure</c> instead of propagating exceptions.
+/// </summary>
+public sealed class RoslynVirtualTagEvaluatorTests
+{
+    [Fact]
+    public void Evaluate_simple_addition_returns_summed_value()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            virtualTagId: "vt-sum",
+            expression: "return (int)ctx.GetTag(\"a\").Value + (int)ctx.GetTag(\"b\").Value;",
+            dependencies: new Dictionary<string, object?> { ["a"] = 10, ["b"] = 32 });
+
+        result.Success.ShouldBeTrue(result.Reason);
+        result.Value.ShouldBe(42);
+    }
+
+    [Fact]
+    public void Evaluate_caches_compiled_expression_across_calls()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+        const string expr = "return (int)ctx.GetTag(\"x\").Value * 2;";
+
+        var first = sut.Evaluate("vt-cache", expr, new Dictionary<string, object?> { ["x"] = 5 });
+        var second = sut.Evaluate("vt-cache", expr, new Dictionary<string, object?> { ["x"] = 7 });
+
+        first.Success.ShouldBeTrue(first.Reason);
+        first.Value.ShouldBe(10);
+        second.Success.ShouldBeTrue(second.Reason);
+        second.Value.ShouldBe(14);
+    }
+
+    [Fact]
+    public void Evaluate_compile_error_returns_Failure_with_reason()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate("vt-bad", "this is not valid C#;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason.ShouldNotBeNull();
+        result.Reason.ShouldContain("compile");
+    }
+
+    [Fact]
+    public void Evaluate_runtime_exception_returns_Failure_with_reason()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            virtualTagId: "vt-div0",
+            expression: "int a = 0; return 1 / a;",
+            dependencies: new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason.ShouldNotBeNull();
+        result.Reason.ShouldContain("threw");
+    }
+
+    [Fact]
+    public void Evaluate_empty_expression_returns_Failure()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        sut.Evaluate("vt-empty", "", new Dictionary<string, object?>()).Success.ShouldBeFalse();
+        sut.Evaluate("vt-empty", "   ", new Dictionary<string, object?>()).Success.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Evaluate_after_dispose_returns_Failure()
+    {
+        var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+        sut.Dispose();
+
+        var result = sut.Evaluate("vt", "return 1;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason!.ShouldContain("disposed");
+    }
+}