feat: implement AI-driven knowledge extraction service with semantic caching and persistent storage

src/NexusReader.Infrastructure/Helpers/ContentHasher.cs

@@ -0,0 +1,37 @@
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.RegularExpressions;
+
+namespace NexusReader.Infrastructure.Helpers;
+
+public static class ContentHasher
+{
+    public static string ComputeHash(string input)
+    {
+        if (string.IsNullOrWhiteSpace(input))
+        {
+            return string.Empty;
+        }
+
+        var normalizedInput = Normalize(input);
+        
+        var inputBytes = Encoding.UTF8.GetBytes(normalizedInput);
+        var hashBytes = SHA256.HashData(inputBytes);
+        
+        return Convert.ToHexString(hashBytes).ToLowerInvariant();
+    }
+
+    public static string Normalize(string input)
+    {
+        if (string.IsNullOrWhiteSpace(input))
+        {
+            return string.Empty;
+        }
+
+        // Trim and collapse all consecutive whitespace characters into a single space
+        var normalized = Regex.Replace(input.Trim(), @"\s+", " ");
+        
+        // Convert to lower-case as AI analysis is generally not case-sensitive for concepts/quizzes
+        return normalized.ToLowerInvariant();
+    }
+}