fix: ISO uses kernel-direct — BOOTX64.EFI is the kernel, no splash
ISOs built by docker-build.sh and downloaded via os.mjs web were using
the chainloader layout (splash.efi as BOOTX64.EFI + KERNEL.EFI). This
fails on machines like Yoga 11e where splash.efi can't load. Now the
ISO puts the kernel directly as BOOTX64.EFI, matching the USB flash
layout. Balena Etcher flashed images will boot on all UEFI machines.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>