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>

src/PiiRedaction.Core/Redaction/PlaceholderPiiRedactor.cs

@@ -0,0 +1,58 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Redaction;
+
+public sealed class PlaceholderPiiRedactor : IPiiRedactor
+{
+    public RedactionResult Redact(string text, IReadOnlyList<PiiEntity> entities)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+        ArgumentNullException.ThrowIfNull(entities);
+
+        var placeholderByValue = new Dictionary<string, string>(StringComparer.Ordinal);
+        var counters = new Dictionary<PiiEntityType, int>();
+        var orderedEntities = entities.OrderByDescending(entity => entity.StartIndex).ToList();
+        var sanitized = text;
+
+        foreach (var entity in orderedEntities)
+        {
+            var mapKey = CreateValueKey(entity);
+            if (!placeholderByValue.TryGetValue(mapKey, out var placeholder))
+            {
+                counters.TryGetValue(entity.Type, out var count);
+                count++;
+                counters[entity.Type] = count;
+                placeholder = $"<{ToPlaceholderPrefix(entity.Type)}_{count}>";
+                placeholderByValue[mapKey] = placeholder;
+            }
+
+            sanitized = string.Concat(
+                sanitized.AsSpan(0, entity.StartIndex),
+                placeholder,
+                sanitized.AsSpan(entity.EndIndex));
+        }
+
+        var placeholderMap = placeholderByValue
+            .ToDictionary(pair => pair.Value, pair => pair.Key.Split('|', 2)[1], StringComparer.Ordinal);
+
+        return new RedactionResult(sanitized, placeholderMap);
+    }
+
+    private static string CreateValueKey(PiiEntity entity) =>
+        $"{entity.Type}|{entity.Value}";
+
+    private static string ToPlaceholderPrefix(PiiEntityType type) => type switch
+    {
+        PiiEntityType.Person => "PERSON",
+        PiiEntityType.Email => "EMAIL",
+        PiiEntityType.Phone => "PHONE",
+        PiiEntityType.Pan => "PAN",
+        PiiEntityType.Aadhaar => "AADHAAR",
+        PiiEntityType.CreditCard => "CREDIT_CARD",
+        PiiEntityType.LoanNumber => "LOAN_NUMBER",
+        PiiEntityType.CustomerId => "CUSTOMER_ID",
+        PiiEntityType.AccountNumber => "ACCOUNT_NUMBER",
+        _ => type.ToString().ToUpperInvariant()
+    };
+}