fix: ACBOOT uses kernel-direct mode — full kernel as BOOTX64.EFI
The flash-helper was using kernel-only mode for ACBOOT, which puts the
kernel as KERNEL.EFI but removes BOOTX64.EFI. UEFI firmware looks for
BOOTX64.EFI as fallback and fails when it's missing.
Switch ACBOOT to kernel-direct: full kernel (with embedded initramfs)
as BOOTX64.EFI. Works on all PC firmware. ACEFI keeps systemd-boot
chain for Macs. Also includes BOOTIA32.EFI, enhanced boot diagnostics,
and VSCode extension OTA stage improvements.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>