review fixes: rename CallingConvention→CallConvention, seal, edge cases

HIGH: rename CallingConvention to CallConvention to avoid BCL collision
 with System.Runtime.InteropServices.CallingConvention.

FIXES:
- checked(uint) casts for x86 pointer truncation (ArgumentOverflow)
- checked distance for E8 rel32 range (>2 GiB → throw)
- add esp, imm32 (81 /0 id) when cleanup > 127 bytes
- pointerSize validation (throw on != 4 and != 8)
- switch default: throw on unknown convention
- track argIndex instead of args[1..] slicing
- EmitMovRegImm32 helper (avoids manual ip tracking bugs)
- seal StubAssembler
- IAssembler doc: note BuildCallStub is StubAssembler-specific
- thiscall 0-args throws test; fastcall 0-args is valid
- update remote-execution spec example to CallConvention.Cdecl

All passing (total: 93).

WhiteMagic/Assembly/CallConvention.cs

@@ -0,0 +1,21 @@
+namespace WhiteMagic.Assembly;
+
+/// <summary>
+/// x86/x86-64 calling conventions for call-stub generation.
+/// Named <c>CallConvention</c> (not <c>CallingConvention</c>) to avoid ambiguity with
+/// <see cref="System.Runtime.InteropServices.CallingConvention"/>.
+/// </summary>
+public enum CallConvention
+{
+    /// <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,
+}