Add runtime x64 stack-alignment regression test

A non-faulting probe payload returns (rsp+8)&15 from the callee, which is 0
only when the stub delivers callee entry rsp ≡ 8 (mod 16) per the Microsoft
x64 ABI. Proves the stub frame alignment end-to-end through CreateRemoteThread
without risking a #GP that would crash the in-process test host. Verified to
fail against the old fixed-0x20 frame.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

WhiteMagicTest/Execution/RemoteThreadExecutorTests.cs

@@ -56,6 +56,15 @@ public sealed class RemoteThreadExecutorTests
     // ret
     private static readonly byte[] PointSumPayload = [0x8B, 0x01, 0x03, 0x41, 0x04, 0xC3];
 
+    // Measures the callee's entry stack alignment without faulting.
+    // mov eax, esp
+    // add eax, 8
+    // and eax, 0x0F
+    // ret
+    // Returns (rsp + 8) & 15, which is 0 iff callee entry rsp ≡ 8 (mod 16) — the
+    // Microsoft x64 ABI guarantee the stub must deliver.
+    private static readonly byte[] AlignProbePayload = [0x89, 0xE0, 0x83, 0xC0, 0x08, 0x83, 0xE0, 0x0F, 0xC3];
+
     [StructLayout(LayoutKind.Sequential)]
     private struct Point
     {
@@ -87,6 +96,20 @@ public sealed class RemoteThreadExecutorTests
         Assert.Equal(15, result);
     }
 
+    [Fact]
+    public void Execute_delivers_16byte_aligned_stack_to_callee()
+    {
+        if (!Environment.Is64BitProcess)
+        {
+            return;
+        }
+
+        // Stub entry rsp ≡ 8 → sub rsp, K (K ≡ 8) → rsp ≡ 0 → call → callee entry rsp ≡ 8.
+        // So (rsp + 8) & 15 == 0 when the frame math is right; a bad K (e.g. 0x20) yields 8.
+        int misalign = RunPayload(AlignProbePayload, CallConvention.Cdecl);
+        Assert.Equal(0, misalign);
+    }
+
     [Fact]
     public void Execute_marshals_string_as_utf8_pointer()
     {