fix: allocate dedicated heap kernel stack for PID 0 (idle task)

fix: allocate dedicated heap kernel stack for PID 0 (idle task)

PID 0 previously used the boot stack from assembly (_stack_top),
which is not heap-managed. This caused two issues:
- TSS esp0 was not updated when switching to PID 0 (kernel_stack
  was NULL, so the guard in schedule() skipped the update)
- If PID 0 were ever reaped or its stack freed, it would corrupt
  memory since the boot stack is not a kmalloc'd block

Now process_init() allocates a 4KB kernel stack via kmalloc and
sets TSS esp0 to its top, matching the pattern used by all other
processes.