refactor: decouple struct process from arch-specific struct registers

refactor: decouple struct process from arch-specific struct registers

- Replace embedded 'struct registers user_regs' with opaque uint8_t user_regs[ARCH_REGS_SIZE]
- Add include/arch_types.h dispatcher and include/arch/x86/arch_types.h (ARCH_REGS_SIZE=64)
- Change arch_regs_set_retval, arch_regs_set_ustack, hal_usermode_enter_regs, arch_sigreturn
  to accept void* instead of struct registers* — arch implementations cast internally
- process_fork_create and process_clone_create now take const void* child_regs
- Remove #include interrupts.h from process.h, arch_process.h, hal/usermode.h, arch_signal.h
- process.h is now fully architecture-agnostic (no x86 register names visible)

20/20 smoke, cppcheck clean