Fix 32-bit host context APIs and ExternalReader bitness detection

- Add GetThreadContext/SetThreadContext overloads accepting Context32 so a
  32-bit process on a native 32-bit OS can capture x86 thread context.
- DllInjector.InjectWithThreadHijack now selects the context API based on
  both process bitness and OS bitness:
  * 64-bit process -> native 64-bit context
  * 32-bit process on 64-bit OS -> WOW64 context
  * 32-bit process on 32-bit OS -> native x86 context
- ExternalReader now validates that the caller supplied
  ProcessAccess.QueryInformation, and surfaces any IsWow64Process failure
  instead of silently falling back to host bitness.

Tests: 207 passing, 4 skipped.

WhiteMagic/Native/NativeMethods.cs

@@ -138,6 +138,20 @@ internal static partial class NativeMethods
         SafeMemoryHandle thread,
         ref Context64 context);
 
+    /// <summary>Sets a 32-bit thread context (x86 or WOW64).</summary>
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool SetThreadContext(
+        SafeMemoryHandle thread,
+        ref Context32 context);
+
+    /// <summary>Gets a 32-bit thread context (x86 or WOW64).</summary>
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool GetThreadContext(
+        SafeMemoryHandle thread,
+        ref Context32 context);
+
     /// <summary>Sets a 32-bit (WOW64) thread context.</summary>
     [LibraryImport("kernel32.dll", SetLastError = true)]
     [return: MarshalAs(UnmanagedType.Bool)]