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.

WhiteMagic/Thread/FrozenThread.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace WhiteMagic.Thread;
+
+/// <summary>
+/// A disposable scope that tracks a set of threads frozen by <see cref="ThreadFactory.Freeze"/>.
+/// Disposing the scope resumes exactly those threads, in reverse order, even if the guarded
+/// body throws.
+/// </summary>
+public sealed class FrozenThread : IDisposable
+{
+    private readonly IReadOnlyList<RemoteThread> _threads;
+    private bool _disposed;
+
+    internal FrozenThread(IReadOnlyList<RemoteThread> threads)
+    {
+        _threads = threads ?? throw new ArgumentNullException(nameof(threads));
+    }
+
+    /// <summary>The threads suspended by this freeze scope.</summary>
+    public IEnumerable<RemoteThread> Threads => _threads;
+
+    /// <summary>
+    /// Resumes the frozen threads in reverse order. The original call is responsible for
+    /// disposing the <see cref="RemoteThread"/> instances afterwards.
+    /// </summary>
+    public void Dispose()
+    {
+        if (_disposed)
+            return;
+
+        _disposed = true;
+
+        foreach (RemoteThread thread in _threads.Reverse())
+        {
+            try
+            {
+                thread.Resume();
+            }
+            catch
+            {
+                // Resume-on-dispose is best-effort; callers keep the thread handles so
+                // they can diagnose or recover separately.
+            }
+        }
+    }
+}