Fix data corruption in ReadBytesAt and full scan

- Fixed ReadBytesAt to read length prefix before data (matching AppendBytes format)
- Fixed full scan to handle length-prefixed records correctly
- All data fields now show correct values instead of corrupted 16777216
- Tests improved from 160/183 to 162/183 passing
- Demo CLI now works correctly with all 10,000 JOIN rows displaying properly

Co-authored-by: MPCoreDeveloper <37024522+MPCoreDeveloper@users.noreply.github.com>

Author: Copilot

SharpCoreDB/DataStructures/Table.cs

@@ -193,15 +193,33 @@ private List<Dictionary<string, object>> SelectInternal(string? where, string? o
                 using var reader = new BinaryReader(ms);
                 while (ms.Position < ms.Length)
                 {
-                    var row = new Dictionary<string, object>();
-                    bool valid = true;
-                    for (int i = 0; i < this.Columns.Count; i++)
+                    try
                     {
-                        try { row[this.Columns[i]] = this.ReadTypedValue(reader, this.ColumnTypes[i]); }
-                        catch { valid = false; break; }
+                        // Read the length prefix that was written by AppendBytes
+                        int recordLength = reader.ReadInt32();
+                        
+                        // Read the record data
+                        var recordData = reader.ReadBytes(recordLength);
+                        
+                        // Deserialize the record
+                        using var recordMs = new MemoryStream(recordData);
+                        using var recordReader = new BinaryReader(recordMs);
+                        
+                        var row = new Dictionary<string, object>();
+                        bool valid = true;
+                        for (int i = 0; i < this.Columns.Count; i++)
+                        {
+                            try { row[this.Columns[i]] = this.ReadTypedValue(recordReader, this.ColumnTypes[i]); }
+                            catch { valid = false; break; }
+                        }
+                        if (valid && (string.IsNullOrEmpty(where) || EvaluateWhere(row, where)))
+                            results.Add(row);
+                    }
+                    catch
+                    {
+                        // End of file or corrupted record
+                        break;
                     }
-                    if (valid && (string.IsNullOrEmpty(where) || EvaluateWhere(row, where)))
-                        results.Add(row);
                 }
             }
         }

SharpCoreDB/Services/Storage.cs

@@ -199,18 +199,20 @@ public long AppendBytes(string path, byte[] data)
 
         using var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read);
         fs.Seek(position, SeekOrigin.Begin);
-        var buffer = new byte[maxLength];
-        int bytesRead = fs.Read(buffer, 0, maxLength);
-        if (bytesRead == 0)
+        using var reader = new BinaryReader(fs);
+        
+        // Read the length prefix that was written by AppendBytes
+        int length = reader.ReadInt32();
+        
+        // Read the actual data
+        int bytesToRead = Math.Min(length, maxLength);
+        var buffer = reader.ReadBytes(bytesToRead);
+        
+        if (buffer.Length == 0)
         {
             return null;
         }
 
-        if (bytesRead < maxLength)
-        {
-            Array.Resize(ref buffer, bytesRead);
-        }
-
         var effectiveNoEncrypt = noEncrypt || this.noEncryption;
         if (effectiveNoEncrypt)
         {