Add PII redaction POC for secure LLM prompting.

Implements detect-redact-sanitize pipeline with regex, domain rules, and ONNX NER before the LLM boundary, plus NUnit tests and Xenovex push documentation.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/TestSupport.Shared/RealNerModelFixture.cs

@@ -0,0 +1,42 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Infrastructure.Onnx;
+
+namespace PiiRedaction.Tests.Shared;
+
+/// <summary>
+/// Reuses a single <see cref="OnnxNerModelRunner"/> per fixture for performance.
+/// Skips all tests in the class when the ONNX model is missing or cannot be loaded.
+/// </summary>
+public abstract class RealNerModelFixture
+{
+    protected OnnxNerModelRunner Runner { get; private set; } = null!;
+
+    protected string ModelPath { get; private set; } = null!;
+
+    [OneTimeSetUp]
+    public void OneTimeSetUpRealModel()
+    {
+        ModelPath = RealNerModelPaths.ResolveRepoModelPath();
+        if (!File.Exists(ModelPath))
+        {
+            Assert.Ignore(RealNerModelPaths.ModelMissingMessage);
+        }
+
+        var options = Options.Create(new PiiRedactionOptions { OnnxModelPath = ModelPath });
+        Runner = new OnnxNerModelRunner(options, NullLogger<OnnxNerModelRunner>.Instance);
+
+        if (!Runner.IsModelAvailable)
+        {
+            Runner.Dispose();
+            Assert.Ignore(RealNerModelPaths.ModelMissingMessage);
+        }
+    }
+
+    [OneTimeTearDown]
+    public void OneTimeTearDownRealModel()
+    {
+        Runner?.Dispose();
+    }
+}