Fix thread namespace collision and tighten executable stub allocation

Fully qualifies System.Threading.Thread in DllInjector after introducing the WhiteMagic.Thread namespace, and replaces the broken+too-small near-allocation loop with a symmetric +/-2 GiB search so the x64 call stub always lands within rel32 range.

WhiteMagicTest/Injection/DllInjectorTests.cs

@@ -1,4 +1,5 @@
 using System.Runtime.InteropServices;
+using Thread = System.Threading.Thread;
 using WhiteMagic;
 using WhiteMagic.Injection;
 using WhiteMagic.Native;
@@ -69,7 +70,7 @@ public class DllInjectorTests
         int osThreadId = 0;
         Exception? threadError = null;
 
-        var helper = new Thread(() =>
+        var helper = new System.Threading.Thread(() =>
         {
             try
             {
@@ -80,7 +81,7 @@ public class DllInjectorTests
                 // also be stopped once its original context is restored.
                 while (!stopEvent.IsSet)
                 {
-                    Thread.Sleep(10);
+                    System.Threading.Thread.Sleep(10);
                 }
             }
             catch (Exception ex)