refactor: move sigframe/sigreturn from syscall.c to arch/x86/signal.c

refactor: move sigframe/sigreturn from syscall.c to arch/x86/signal.c

- New include/arch/x86/signal.h: shared struct sigframe + SIGFRAME_MAGIC
- New include/arch_signal.h: arch-agnostic arch_sigreturn() prototype
- New src/arch/x86/signal.c: x86 sigreturn implementation (eflags sanitize,
  CS/SS ring3 validation, IOPL clear)
- src/arch/x86/idt.c: use shared arch/x86/signal.h instead of local copy
- src/kernel/syscall.c: remove x86-specific sigframe struct and sigreturn_impl,
  call arch_sigreturn() via generic void* interface

No x86 signal frame knowledge remains in generic kernel code.