refactor: extract EFI_DIR constant for the install-time EFI mount point

The literal /mnt/efi appeared at 17 sites across installer/archangel and installer/lib/btrfs.sh. Renaming it (or pointing tests at a different mount) meant touching every site and risking incomplete sweeps. One canonical name in installer/lib/common.sh now backs every reference.

EFI_DIR has no trailing slash so the three expansion patterns in the codebase compose cleanly. Bare ($EFI_DIR), sub-path ($EFI_DIR/EFI/ZBM), and the index-suffix used by install_grub_all_efi for secondary EFI mounts (${EFI_DIR}${i}). The sync_efi_partitions staging path also moves from the literal /mnt/efi_sync to ${EFI_DIR}_sync, so it follows EFI_DIR if anyone ever changes the base.

Two follow-ups filed as separate :techdebt: items. MNTPOINT=/mnt extraction across the 50+ /mnt/... sites (pacstrap, arch-chroot, fstab writes), and the related /mnt${chroot_efi_dir} composition pattern at btrfs.sh:681-682. Both ship together when MNTPOINT lands.

Verified: bats 134 → 135 (+1 pinning EFI_DIR=/mnt/efi). Lint clean. All four expansion patterns smoke-tested at runtime and produce the original literal byte-for-byte. VM run skipped, pure constant substitution with zero behavior change.

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