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/ExternalReader.cs

@@ -37,6 +37,13 @@ public sealed class ExternalReader : MemoryBase
     public ExternalReader(System.Diagnostics.Process process, ProcessAccess desiredAccess = DefaultAccess)
     {
         _processId = process.Id;
+        if ((desiredAccess & ProcessAccess.QueryInformation) == 0)
+        {
+            throw new ArgumentException(
+                "ExternalReader requires ProcessAccess.QueryInformation to determine target bitness.",
+                nameof(desiredAccess));
+        }
+
         _handle = NativeMethods.OpenProcess(desiredAccess, false, _processId);
         if (_handle.IsInvalid)
         {
@@ -46,11 +53,12 @@ public sealed class ExternalReader : MemoryBase
         }
 
         // Derive target bitness. A 64-bit host sees a 32-bit target as WOW64.
-        // A 32-bit host can only open 32-bit targets. If the API fails, fall
-        // back to the current process bitness (self-open path).
+        // A 32-bit host can only open 32-bit targets.
         if (!NativeMethods.IsWow64Process(_handle, out bool wow64))
         {
-            wow64 = false;
+            int error = Marshal.GetLastPInvokeError();
+            throw new InvalidOperationException(
+                $"IsWow64Process failed for PID {_processId}: error {error}.");
         }
 
         _is64Bit = Environment.Is64BitProcess && !wow64;