task 2.5-2.6: string Read/Write with encoding tests

Add 8 tests for string IO: ASCII/UTF8/Unicode round-trip, null-terminator
stop, max-length truncation, auto-append of null terminator, empty string.
Fix ReadString null-terminator detection for multi-byte encodings (UTF-16):
decode string first, then find \0 in characters not bytes.

All passing (total: 38).

WhiteMagic/MemoryBase.cs

@@ -141,12 +141,13 @@ public abstract class MemoryBase : IDisposable
     public virtual string ReadString(IntPtr address, Encoding encoding, int maxLength = 512, bool relative = false)
     {
         byte[] buffer = ReadBytes(address, maxLength, relative);
-        int nullIndex = Array.IndexOf<byte>(buffer, 0);
+        string decoded = encoding.GetString(buffer);
+        int nullIndex = decoded.IndexOf('\0');
         if (nullIndex >= 0)
         {
-            return encoding.GetString(buffer, 0, nullIndex);
+            return decoded[..nullIndex];
         }
-        return encoding.GetString(buffer);
+        return decoded;
     }
 
     /// <summary>Writes a null-terminated string to the target address.</summary>