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.

2 files changed, 16 insertions, 6 deletions

diff --git a/installer/lib/btrfs.sh b/installer/lib/btrfs.sh
index f1cfaac..09127b2 100644
--- a/installer/lib/btrfs.sh
+++ b/installer/lib/btrfs.sh
@@ -567,8 +567,8 @@ configure_grub() {
     step "Configuring GRUB Bootloader"
 
     # Mount EFI partition
-    mkdir -p /mnt/efi
-    mount "$efi_partition" /mnt/efi
+    mkdir -p $EFI_DIR
+    mount "$efi_partition" $EFI_DIR
 
     # Configure GRUB defaults for btrfs
     info "Setting GRUB configuration..."
@@ -624,7 +624,7 @@ EOF
 
     # Create grub directory on EFI partition
     # GRUB modules on FAT32 EFI partition avoid btrfs subvolume path issues
-    mkdir -p /mnt/efi/grub
+    mkdir -p $EFI_DIR/grub
 
     # Install GRUB with boot-directory on EFI partition
     info "Installing GRUB to EFI partition..."
@@ -665,12 +665,12 @@ install_grub_all_efi() {
     for efi_part in "${efi_partitions[@]}"; do
         # First EFI at /efi (already mounted), subsequent at /efi2, /efi3, etc.
         local chroot_efi_dir="/efi"
-        local mount_point="/mnt/efi"
+        local mount_point="$EFI_DIR"
         local bootloader_id="GRUB"
 
         if [[ $i -gt 1 ]]; then
             chroot_efi_dir="/efi${i}"
-            mount_point="/mnt/efi${i}"
+            mount_point="$EFI_DIR${i}"
             bootloader_id="GRUB-disk${i}"
 
             # Mount secondary EFI partitions
@@ -887,7 +887,7 @@ btrfs_cleanup() {
     sync
 
     # Unmount EFI first
-    umount /mnt/efi 2>/dev/null || true
+    umount $EFI_DIR 2>/dev/null || true
 
     # Unmount all btrfs subvolumes (reverse order)
     for ((i=${#BTRFS_SUBVOLS[@]}-1; i>=0; i--)); do
diff --git a/installer/lib/common.sh b/installer/lib/common.sh
index 98220fa..d375a9c 100644
--- a/installer/lib/common.sh
+++ b/installer/lib/common.sh
@@ -3,6 +3,16 @@
 # Source this file: source "$(dirname "$0")/lib/common.sh"
 
 #############################
+# Path Constants
+#############################
+
+# Mount point for the primary EFI partition during install. Sub-paths
+# compose with ${EFI_DIR}/...; secondary EFI partitions in multi-disk
+# layouts use ${EFI_DIR}${i} (no trailing slash, so the index appends
+# cleanly).
+EFI_DIR="/mnt/efi"
+
+#############################
 # Output Functions
 #############################