Add thread control surfaces

Adds RemoteThread, ThreadFactory (enumeration, main-thread selection, get-by-id), and FrozenThread scoped freeze. Supports suspend/resume, 32/64-bit context round-trip, TEB query, and reverse-order resume on dispose. Closes section 2 of add-thread-region-finder.

WhiteMagicTest/Thread/ThreadFactoryTests.cs

@@ -0,0 +1,61 @@
+using System.Linq;
+using System.Threading;
+using SysThread = System.Threading.Thread;
+using WhiteMagic;
+using WhiteMagic.Native;
+using WhiteMagic.Thread;
+using Xunit;
+
+namespace WhiteMagicTest.Thread;
+
+/// <summary>
+/// Tests for <see cref="ThreadFactory"/> enumeration and main-thread selection.
+/// </summary>
+public sealed class ThreadFactoryTests
+{
+    [Fact]
+    public void Enumerate_returns_only_target_threads()
+    {
+        using var magic = Magic.OpenInProcess();
+        var factory = new ThreadFactory(magic.Memory);
+
+        int currentOsId = (int)NativeMethods.GetCurrentThreadId();
+        var ids = factory.Enumerate().Select(t => t.Id).ToList();
+
+        Assert.True(ids.Count > 0);
+        Assert.Contains(currentOsId, ids);
+    }
+
+    [Fact]
+    public void GetThreadById_returns_matching_thread()
+    {
+        using var magic = Magic.OpenInProcess();
+        var factory = new ThreadFactory(magic.Memory);
+
+        int currentOsId = (int)NativeMethods.GetCurrentThreadId();
+        using RemoteThread thread = factory.GetThreadById(currentOsId);
+        Assert.Equal(currentOsId, thread.Id);
+    }
+
+    [Fact]
+    public void GetThreadById_throws_for_nonexistent_thread()
+    {
+        using var magic = Magic.OpenInProcess();
+        var factory = new ThreadFactory(magic.Memory);
+
+        Assert.Throws<InvalidOperationException>(() => factory.GetThreadById(0x7FFFFFFF));
+    }
+
+    [Fact]
+    public void MainThread_returns_a_thread_belonging_to_the_target()
+    {
+        using var magic = Magic.OpenInProcess();
+        var factory = new ThreadFactory(magic.Memory);
+
+        using RemoteThread main = factory.MainThread;
+        Assert.NotNull(main);
+
+        var ids = factory.Enumerate().Select(t => t.Id).ToList();
+        Assert.Contains(main.Id, ids);
+    }
+}