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).

WhiteMagic/Assembly/StubAssembler.cs

@@ -14,8 +14,6 @@ namespace WhiteMagic.Assembly;
 public class StubAssembler : IAssembler
 {
     /// <inheritdoc />
-    /// <exception cref="NotSupportedException">Always thrown. StubAssembler does
-    /// not parse text assembly; use IcedAssembler for that.</exception>
     public byte[] Assemble(string assemblyText, ulong origin = 0)
     {
         throw new NotSupportedException(
@@ -25,13 +23,8 @@ public class StubAssembler : IAssembler
 
     // ── Emit primitives ────────────────────────────────────────────────────
 
-    /// <summary>Appends a single byte to <paramref name="buffer"/>.</summary>
-    public void EmitU8(List<byte> buffer, byte value)
-    {
-        buffer.Add(value);
-    }
+    public void EmitU8(List<byte> buffer, byte value) => buffer.Add(value);
 
-    /// <summary>Appends <paramref name="value"/> as 4 little-endian bytes.</summary>
     public void EmitU32(List<byte> buffer, uint value)
     {
         buffer.Add((byte)value);
@@ -40,10 +33,86 @@ public class StubAssembler : IAssembler
         buffer.Add((byte)(value >> 24));
     }
 
-    /// <summary>Appends <paramref name="value"/> as 8 little-endian bytes.</summary>
     public void EmitU64(List<byte> buffer, ulong value)
     {
         EmitU32(buffer, (uint)value);
         EmitU32(buffer, (uint)(value >> 32));
     }
+
+    // ── Call-stub builders ─────────────────────────────────────────────────
+
+    public byte[] BuildCallStub(IntPtr stubAddress, IntPtr targetAddress,
+        uint[] arguments, int pointerSize, CallingConvention convention)
+    {
+        var buffer = new List<byte>(64);
+
+        if (pointerSize == 4)
+            BuildX86Stub(buffer, (uint)stubAddress, (uint)targetAddress, arguments, convention);
+        else
+            BuildX64Stub(buffer, (ulong)stubAddress, (ulong)targetAddress, arguments);
+
+        return buffer.ToArray();
+    }
+
+    private void BuildX86Stub(List<byte> buffer, uint stubAddr,
+        uint target, uint[] args, CallingConvention convention)
+    {
+        uint current = stubAddr;
+
+        switch (convention)
+        {
+            case CallingConvention.Thiscall when args.Length >= 1:
+                buffer.Add(0xB9);         // mov ecx, arg0
+                EmitU32(buffer, args[0]);
+                current += 5;
+                args = args[1..];
+                break;
+
+            case CallingConvention.Fastcall:
+                if (args.Length >= 1)
+                {
+                    buffer.Add(0xB9);     // mov ecx, arg0
+                    EmitU32(buffer, args[0]);
+                    current += 5;
+                    args = args[1..];
+                }
+                if (args.Length >= 1)
+                {
+                    buffer.Add(0xBA);     // mov edx, arg1
+                    EmitU32(buffer, args[0]);
+                    current += 5;
+                    args = args[1..];
+                }
+                break;
+        }
+
+        // Push remaining args in reverse order
+        for (int i = args.Length - 1; i >= 0; i--)
+        {
+            buffer.Add(0x68);             // push imm32
+            EmitU32(buffer, args[i]);
+            current += 5;
+        }
+
+        // call rel32
+        uint rel32 = target - (current + 5);
+        buffer.Add(0xE8);
+        EmitU32(buffer, rel32);
+
+        // Caller cleanup (cdecl only)
+        if (convention == CallingConvention.Cdecl && args.Length > 0)
+        {
+            buffer.Add(0x83);             // add esp, imm8
+            buffer.Add(0xC4);
+            buffer.Add((byte)(args.Length * 4));
+        }
+
+        buffer.Add(0xC3);                 // ret
+    }
+
+    private static void BuildX64Stub(List<byte> buffer, ulong stubAddr,
+        ulong target, uint[] args)
+    {
+        throw new NotImplementedException("x64 stubs (task 3.7-3.8)");
+    }
 }