Replace GRUB with ZFSBootMenu bootloader

This is a major change that replaces the GRUB bootloader with ZFSBootMenu,
providing native ZFS boot environment support.

Key changes:
- EFI partition reduced from 1GB to 512MB (only holds ZFSBootMenu)
- EFI now mounts at /efi instead of /boot
- Kernel and initramfs live on ZFS root (enables snapshot boot with matching kernel)
- Downloads pre-built ZFSBootMenu EFI binary from get.zfsbootmenu.org
- Creates EFI boot entries for all disks in multi-disk configurations
- Syncs ZFSBootMenu to all EFI partitions for redundancy
- Sets org.zfsbootmenu:commandline on zroot/ROOT for kernel cmdline inheritance
- Sets bootfs pool property for default boot environment
- AMD GPU workarounds (pg_mask, cwsr_enable) added to kernel cmdline when AMD detected

Deleted GRUB snapshot tooling (no longer needed):
- custom/grub-zfs-snap
- custom/40_zfs_snapshots
- custom/zz-grub-zfs-snap.hook
- custom/zfs-snap-prune

Updated helper scripts:
- zfssnapshot: removed grub-zfs-snap call, shows ZFSBootMenu tip
- zfsrollback: removed grub-zfs-snap call, notes auto-detection

Tested configurations:
- Single disk installation
- 2-disk mirror (mirror-0)
- 3-disk RAIDZ1 (raidz1-0)
- All boot correctly with ZFSBootMenu

1 files changed, 2 insertions, 7 deletions

diff --git a/custom/zfssnapshot b/custom/zfssnapshot
index 1fa7e3b..749ea5a 100755
--- a/custom/zfssnapshot
+++ b/custom/zfssnapshot
@@ -101,10 +101,5 @@ done
 
 echo ""
 echo "Snapshot complete. Verify with: zfs list -t snapshot | grep $snapshot_name"
-
-# Update GRUB boot menu if grub-zfs-snap is available
-if command -v grub-zfs-snap &> /dev/null; then
-    echo ""
-    echo "Updating GRUB boot menu..."
-    grub-zfs-snap
-fi
+echo ""
+echo "To boot from this snapshot: reboot and press Ctrl+D at ZFSBootMenu"