fix: flash helper uses BOOTX64.EFI as staged kernel path
media-layout.sh stages the kernel as BOOTX64.EFI (kernel-direct mode)
but flash-helper-runner.sh still referenced KERNEL.EFI, causing
"cannot stat KERNEL.EFI" during USB flash. Updated all staged-root
reads and verification checks to match the current layout.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>