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.

WhiteMagicTest/MemoryHardeningTests.cs

@@ -151,6 +151,15 @@ public class MemoryHardeningTests
         Assert.False(reader.Handle.IsInvalid);
     }
 
+    [Fact]
+    public void ExternalReader_throws_when_query_information_access_missing()
+    {
+        var ex = Assert.Throws<ArgumentException>(() =>
+            new ExternalReader(Process.GetCurrentProcess(), ProcessAccess.VmRead));
+
+        Assert.Equal("desiredAccess", ex.ParamName);
+    }
+
     /// <summary>
     /// A <see cref="MemoryBase"/> that serves bytes from an in-memory buffer and
     /// caps every read to <c>maxChunk</c> bytes, to exercise partial-read handling.