refactor: extract x86 kernel stack setup and register accessors from scheduler to...

refactor: extract x86 kernel stack setup and register accessors from scheduler to arch layer

- New include/arch_process.h: arch-agnostic prototypes for arch_kstack_init(),
  arch_regs_set_retval(), arch_regs_set_ustack()
- New src/arch/x86/arch_process.c: x86 implementation (EFLAGS 0x202, cdecl
  stack frame layout matching context_switch in process.S)
- scheduler.c: process_create_kernel, process_fork_create, process_clone_create
  now use arch_kstack_init() instead of inline x86 stack manipulation
- scheduler.c: process_clone_create uses arch_regs_set_retval/arch_regs_set_ustack
  instead of direct .eax/.useresp access

No x86-specific constants or register names remain in scheduler.c.