fix: use chainloader on ESP — Mac EFI can't load 270MB kernel directly
Mac firmware hangs trying to load 270MB BOOTX64.EFI. Switch ESP back
to chainloader mode: small splash.efi (51KB) loads first, then chains
to KERNEL.EFI. Confirmed: chainloader gets past Mac boot picker.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>