task 3.3-3.4: x86 cdecl stub encoding + CallingConvention enum

Add CallingConvention enum (Cdecl, Stdcall, Thiscall, Fastcall).
Implement BuildCallStub on StubAssembler with x86 cdecl support:
reverse arg push, call rel32, add esp (caller cleanup), ret.
x64 stub is a placeholder (task 3.7-3.8).

3 new tests: 0-arg (call+ret), 1-arg (push+call+cleanup+ret),
2-args (reverse push+call+cleanup+ret). Known byte expectations.
All passing (total: 67).

WhiteMagicTest/StubAssemblerTests.cs

@@ -4,8 +4,8 @@ namespace WhiteMagicTest;
 
 /// <summary>
 /// Tests for <see cref="StubAssembler"/> emit primitives (<see cref="StubAssembler.EmitU8"/>,
-/// <see cref="StubAssembler.EmitU32"/>, <see cref="StubAssembler.EmitU64"/>) and the
-/// <see cref="IAssembler"/> interface contract.
+/// <see cref="StubAssembler.EmitU32"/>, <see cref="StubAssembler.EmitU64"/>) and
+/// calling-convention stub encoding.
 /// </summary>
 public class StubAssemblerTests
 {
@@ -75,4 +75,68 @@ public class StubAssemblerTests
         var sut = Create();
         Assert.Throws<NotSupportedException>(() => sut.Assemble("nop", 0));
     }
+
+    // ── Calling convention: x86 cdecl ──────────────────────────────────────
+
+    [Fact]
+    public void Cdecl_stub_with_zero_args_is_call_then_ret()
+    {
+        var sut = Create();
+        IntPtr stubAddr = (IntPtr)0x10000000;
+        IntPtr target = (IntPtr)0x12345678;
+        uint[] args = [];
+
+        uint rel32 = (uint)target - ((uint)stubAddr + 5);
+        byte[] stub = sut.BuildCallStub(stubAddr, target, args, 4, CallingConvention.Cdecl);
+
+        Assert.Equal(6, stub.Length);
+        Assert.Equal(0xE8, stub[0]);                           // call
+        Assert.Equal(rel32, BitConverter.ToUInt32(stub, 1));   // rel32
+        Assert.Equal(0xC3, stub[5]);                           // ret
+    }
+
+    [Fact]
+    public void Cdecl_stub_one_arg_reverse_push_then_call_then_cleanup()
+    {
+        var sut = Create();
+        IntPtr stubAddr = (IntPtr)0x10000000;
+        IntPtr target = (IntPtr)0x12345678;
+        uint[] args = [0xAABBCCDD];
+
+        uint callAddr = (uint)stubAddr + 5;
+        uint rel32 = (uint)target - (callAddr + 5);
+        byte[] stub = sut.BuildCallStub(stubAddr, target, args, 4, CallingConvention.Cdecl);
+
+        byte[] expected = [
+            0x68, 0xDD, 0xCC, 0xBB, 0xAA, // push 0xAABBCCDD
+            0xE8,
+            (byte)rel32, (byte)(rel32 >> 8), (byte)(rel32 >> 16), (byte)(rel32 >> 24),
+            0x83, 0xC4, 0x04,              // add esp, 4
+            0xC3                            // ret
+        ];
+        Assert.Equal(expected, stub);
+    }
+
+    [Fact]
+    public void Cdecl_stub_two_args_reverse_order()
+    {
+        var sut = Create();
+        IntPtr stubAddr = (IntPtr)0x10000000;
+        IntPtr target = (IntPtr)0x12345678;
+        uint[] args = [0x11111111, 0x22222222];
+
+        uint callAddr = (uint)stubAddr + 10;
+        uint rel32 = (uint)target - (callAddr + 5);
+        byte[] stub = sut.BuildCallStub(stubAddr, target, args, 4, CallingConvention.Cdecl);
+
+        byte[] expected = [
+            0x68, 0x22, 0x22, 0x22, 0x22, // push arg1 (reverse order)
+            0x68, 0x11, 0x11, 0x11, 0x11, // push arg0
+            0xE8,
+            (byte)rel32, (byte)(rel32 >> 8), (byte)(rel32 >> 16), (byte)(rel32 >> 24),
+            0x83, 0xC4, 0x08,              // add esp, 8
+            0xC3
+        ];
+        Assert.Equal(expected, stub);
+    }
 }