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/IAssembler.cs

@@ -6,6 +6,11 @@ namespace WhiteMagic.Assembly;
 /// <see cref="IcedAssembler"/> (Phase 8) handles arbitrary mnemonics via the Iced
 /// library.
 /// </summary>
+/// <remarks>
+/// This seam covers text assembly only (<see cref="Assemble"/>). Call-stub building
+/// (<c>BuildCallStub</c>, <c>EmitU8</c>/<c>EmitU32</c>/<c>EmitU64</c>) is a
+/// <see cref="StubAssembler"/> capability — not all backends need it.
+/// </remarks>
 public interface IAssembler
 {
     /// <summary>