MMIO/UART/PLIC/VIRTIO addresses changed from low VA (0x1xxxxxxx) to
high VA (0xFFFFFFC0_1xxxxxxx), accessible in all page tables via Boot
PT's 1GB leaf PTE — fixes double-fault crash when kernel writes to
UART from user page table context
Remove MMIO MapType::Identical loop in new_kernel() (now redundant)
Replace write_volatile(0x10000000, ...) debug markers with println!
in boot paths; remove markers from hot paths (trap/IRQ/scheduler)