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/CallingConvention.cs

@@ -0,0 +1,19 @@
+namespace WhiteMagic.Assembly;
+
+/// <summary>
+/// x86/x86-64 calling conventions for call-stub generation.
+/// </summary>
+public enum CallingConvention
+{
+    /// <summary>Caller pushes args right-to-left and cleans the stack (x86).</summary>
+    Cdecl,
+
+    /// <summary>Caller pushes args right-to-left; callee cleans the stack (x86).</summary>
+    Stdcall,
+
+    /// <summary>ECX receives the <c>this</c> pointer; remaining args on stack right-to-left; callee cleans (x86).</summary>
+    Thiscall,
+
+    /// <summary>ECX/EDX receive the first two args; remaining on stack right-to-left; callee cleans (x86).</summary>
+    Fastcall,
+}