refactor: merge install_base and install_base_btrfs

Extract the pacstrap package list into pacstrap_packages(filesystem)
in lib/common.sh (common + filesystem-specific). install_base() now
dispatches on FILESYSTEM for both the archzfs-repo-append and the
package list. install_base_btrfs() deleted; install_btrfs() call site
updated to invoke install_base.

Old: 49 + 38 lines of ~95% copy-paste.
New: 32 lines + a 20-line pure helper.

7 bats tests cover: zfs has zfs-dkms/zfs-utils, btrfs has btrfs-progs
+ grub + grub-btrfs + snapper + snap-pac, each flavor excludes the
other's specifics, common packages are in both, unknown filesystem
returns status 1, output is one-per-line. make test: 65/65.

2 files changed, 43 insertions, 69 deletions

diff --git a/installer/archangel b/installer/archangel
index 4dc6689..5719d4f 100755
--- a/installer/archangel
+++ b/installer/archangel
@@ -878,84 +878,30 @@ install_base() {
     pacman-key --init
     pacman-key --populate archlinux
 
-    # Add archzfs repo to pacman.conf for pacstrap
-    # SigLevel=Never: pacstrap -K creates empty keyring where key import fails;
-    # repo is explicitly added and served over HTTPS, GPG adds no real value here
-    if ! grep -q "\[archzfs\]" /etc/pacman.conf; then
-        cat >> /etc/pacman.conf << 'EOF'
+    # ZFS needs the archzfs repo added to pacman.conf for pacstrap.
+    # SigLevel=Never: pacstrap -K creates empty keyring where key import
+    # fails; repo is explicitly added and served over HTTPS, GPG adds no
+    # real value here.
+    if [[ "$FILESYSTEM" == "zfs" ]]; then
+        if ! grep -q "\[archzfs\]" /etc/pacman.conf; then
+            cat >> /etc/pacman.conf << 'EOF'
 
 [archzfs]
 Server = https://github.com/archzfs/archzfs/releases/download/experimental
 SigLevel = Never
 EOF
+        fi
+        info "ZFS will be built from source via DKMS - this ensures kernel compatibility."
     fi
 
     info "Installing base packages (this takes a while)..."
-    info "ZFS will be built from source via DKMS - this ensures kernel compatibility."
-    # Use yes to auto-select defaults for provider prompts
-    yes "" | pacstrap -K /mnt \
-        base \
-        base-devel \
-        linux-lts \
-        linux-lts-headers \
-        linux-firmware \
-        zfs-dkms \
-        zfs-utils \
-        efibootmgr \
-        networkmanager \
-        avahi \
-        nss-mdns \
-        openssh \
-        git \
-        vim \
-        sudo \
-        zsh \
-        nodejs \
-        npm \
-        ttf-dejavu \
-        fzf \
-        wget \
-        inetutils \
-        wireless-regdb
 
-    info "Base system installed."
-}
-
-install_base_btrfs() {
-    step "Installing Base System (Btrfs)"
+    local packages
+    mapfile -t packages < <(pacstrap_packages "$FILESYSTEM") \
+        || error "Unknown filesystem: $FILESYSTEM"
 
-    info "Updating pacman keys..."
-    pacman-key --init
-    pacman-key --populate archlinux
-
-    info "Installing base packages (this takes a while)..."
-    yes "" | pacstrap -K /mnt \
-        base \
-        base-devel \
-        linux-lts \
-        linux-lts-headers \
-        linux-firmware \
-        btrfs-progs \
-        grub \
-        grub-btrfs \
-        efibootmgr \
-        snapper \
-        snap-pac \
-        networkmanager \
-        avahi \
-        nss-mdns \
-        openssh \
-        git \
-        vim \
-        sudo \
-        zsh \
-        nodejs \
-        npm \
-        ttf-dejavu \
-        fzf \
-        wget \
-        inetutils \
-        wireless-regdb
+    # Use yes to auto-select defaults for provider prompts
+    yes "" | pacstrap -K /mnt "${packages[@]}"
 
     info "Base system installed."
 }
@@ -1614,7 +1560,7 @@ install_btrfs() {
     mount "${efi_parts[0]}" /mnt/efi
 
     # Install base system
-    install_base_btrfs
+    install_base
 
     # Configure system
     configure_system
diff --git a/installer/lib/common.sh b/installer/lib/common.sh
index dcaf071..d181e0b 100644
--- a/installer/lib/common.sh
+++ b/installer/lib/common.sh
@@ -57,6 +57,34 @@ require_command() {
 }
 
 #############################
+# Package Selection
+#############################
+
+# Print the pacstrap package list for the given filesystem, one per line.
+# Common packages first, then filesystem-specific ones.
+# Returns 1 for unknown filesystem.
+#
+# Usage: mapfile -t pkgs < <(pacstrap_packages zfs)
+pacstrap_packages() {
+    local fs="$1"
+    local common=(
+        base base-devel
+        linux-lts linux-lts-headers linux-firmware
+        efibootmgr
+        networkmanager avahi nss-mdns openssh
+        git vim sudo zsh nodejs npm
+        ttf-dejavu fzf wget inetutils wireless-regdb
+    )
+    local fs_specific
+    case "$fs" in
+        zfs)   fs_specific=(zfs-dkms zfs-utils) ;;
+        btrfs) fs_specific=(btrfs-progs grub grub-btrfs snapper snap-pac) ;;
+        *)     return 1 ;;
+    esac
+    printf '%s\n' "${common[@]}" "${fs_specific[@]}"
+}
+
+#############################
 # Password / Passphrase Input
 #############################