Guard diagnostic stub builder against argument-count overflow

The x64 frame math (0x20 + 8*stackArgs) and x86 arg buffer size grow with the
argument count. An absurdly large count could overflow int and produce a bogus
or negative frame. Add a MaxArguments (256) bound checked at the public entry —
far above any real calling convention — so the arithmetic stays in range. Add a
test asserting the cap is inclusive and count+1 throws.

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

WhiteMagicTest/StubAssemblerTests.cs

@@ -444,6 +444,20 @@ public class StubAssemblerTests
             Create().BuildCallStub((IntPtr)0x10000000, (IntPtr)0x12345678, [], 4, (CallConvention)99));
     }
 
+    [Fact]
+    public void Too_many_arguments_throws_before_frame_math_overflows()
+    {
+        // Guards the 0x20 + 8*stackArgs frame arithmetic against int overflow.
+        // MaxArguments is honored (accepted) and MaxArguments+1 is rejected.
+        var atCap = new nuint[StubAssembler.MaxArguments];
+        // At the cap the call still builds (cdecl x64), proving the bound is inclusive.
+        _ = Create().BuildCallStub((IntPtr)0x10000000, (IntPtr)0x10001000, atCap, 8, CallConvention.Cdecl);
+
+        var overCap = new nuint[StubAssembler.MaxArguments + 1];
+        Assert.Throws<ArgumentOutOfRangeException>(() =>
+            Create().BuildCallStub((IntPtr)0x10000000, (IntPtr)0x10001000, overCap, 8, CallConvention.Cdecl));
+    }
+
     // ── No-FASM ─────────────────────────────────────────────────────────
 
     [Fact]