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/Models/PiiDetectionSource.cs

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+namespace PiiRedaction.Core.Models;
+
+public enum PiiDetectionSource
+{
+    Regex,
+    Domain,
+    Ner
+}

src/PiiRedaction.Core/Models/PiiEntity.cs

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+namespace PiiRedaction.Core.Models;
+
+public sealed record PiiEntity(
+    PiiEntityType Type,
+    string Value,
+    int StartIndex,
+    int Length,
+    PiiDetectionSource Source,
+    double? Confidence = null)
+{
+    public int EndIndex => StartIndex + Length;
+}

src/PiiRedaction.Core/Models/PiiEntityType.cs

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+namespace PiiRedaction.Core.Models;
+
+public enum PiiEntityType
+{
+    Person,
+    Email,
+    Phone,
+    Pan,
+    Aadhaar,
+    CreditCard,
+    LoanNumber,
+    CustomerId,
+    AccountNumber
+}

src/PiiRedaction.Core/Models/RedactionResult.cs

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+namespace PiiRedaction.Core.Models;
+
+public sealed record RedactionResult(
+    string SanitizedText,
+    IReadOnlyDictionary<string, string> PlaceholderMap);

src/PiiRedaction.Core/Models/SanitizationRequest.cs

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+namespace PiiRedaction.Core.Models;
+
+public sealed record SanitizationRequest(string OriginalPrompt);

src/PiiRedaction.Core/Models/SanitizationResult.cs

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+namespace PiiRedaction.Core.Models;
+
+public sealed record SanitizationResult(
+    string OriginalPrompt,
+    string SanitizedPrompt,
+    IReadOnlyList<PiiEntity> DetectedEntities,
+    RedactionResult Redaction);