refactor: verify GRUB_CMDLINE_LINUX seds via prepend_grub_cmdline_linux helper

Audited the ~10 silent sed -i sites in the installer against the verification-after pattern that landed for sshd_config last session. Triaged each by failure mode.

The two GRUB_CMDLINE_LINUX seds in lib/btrfs.sh have a real silent-failure risk. If /etc/default/grub is missing or malformed and the sed pattern doesn't match, nothing happens. The kernel boots without cryptdevice=. The system can't unlock LUKS at boot. Added prepend_grub_cmdline_linux to lib/common.sh. Same shape as enable_sshd_root_login (sed, then grep, then error if the line wasn't modified). Replaced the two inline seds with helper calls.

The HOOKS= seds in installer/archangel and lib/btrfs.sh (six total) don't need verification. A missing HOOKS= line makes mkinitcpio -P fail loudly downstream, so silent-replace failure can't reach a booted system. Added a one-line audit-rationale comment at each of the three locations so the next reader doesn't re-litigate the decision.

The FILES= sed at lib/btrfs.sh:213 already self-heals via a sed-then-grep-then-append pattern, so no behavior change there. Filed a separate follow-up to lift that pattern into a named helper for clarity.

Bats: 142 → 146. Four new tests in test_common.bats cover normal (empty cmdline, existing cmdline preserved, other lines preserved) and error (missing GRUB_CMDLINE_LINUX line). Lint clean.

3 files changed, 30 insertions, 2 deletions

diff --git a/installer/archangel b/installer/archangel
index 12f1cd0..479b53a 100755
--- a/installer/archangel
+++ b/installer/archangel
@@ -896,6 +896,9 @@ EOF
     #   This ensures NVMe, AHCI, and other storage drivers are always included
     # - Remove fsck: ZFS doesn't use it, avoids confusing error messages
     # - Add zfs: required for ZFS root boot
+    # No sed verification needed: a missing HOOKS= line makes mkinitcpio -P
+    # fail loudly downstream, so silent-replace failure can't reach a booted
+    # system. (Audited 2026-04-27 against silent-sed pattern.)
     sed -i 's/^HOOKS=.*/HOOKS=(base udev microcode modconf kms keyboard keymap consolefont block zfs filesystems)/' /mnt/etc/mkinitcpio.conf
 
     # Get the installed kernel version (not the running kernel)
diff --git a/installer/lib/btrfs.sh b/installer/lib/btrfs.sh
index 09127b2..f704fd7 100644
--- a/installer/lib/btrfs.sh
+++ b/installer/lib/btrfs.sh
@@ -204,6 +204,8 @@ configure_luks_initramfs() {
 
     # Add encrypt hook before filesystems (configure_btrfs_initramfs overwrites
     # this with the final hook list, using sd-encrypt for multi-disk setups)
+    # No sed verification needed: a missing HOOKS= line makes mkinitcpio -P
+    # fail loudly downstream. (Audited 2026-04-27 against silent-sed pattern.)
     sed -i 's/^HOOKS=.*/HOOKS=(base udev microcode modconf kms keyboard keymap consolefont block encrypt filesystems fsck)/' \
         /mnt/etc/mkinitcpio.conf
 
@@ -247,7 +249,8 @@ configure_luks_grub() {
         info "Testing mode: adding cryptkey parameter for automated unlock"
     fi
 
-    sed -i "s|^GRUB_CMDLINE_LINUX=\"|GRUB_CMDLINE_LINUX=\"cryptdevice=UUID=$uuid:$LUKS_MAPPER_NAME:allow-discards ${cryptkey_param}|" \
+    prepend_grub_cmdline_linux \
+        "cryptdevice=UUID=$uuid:$LUKS_MAPPER_NAME:allow-discards ${cryptkey_param}" \
         /mnt/etc/default/grub
 
     info "GRUB configured with cryptdevice parameter and cryptodisk enabled."
@@ -613,7 +616,8 @@ EOF
                     cryptkey_param="cryptkey=rootfs:$LUKS_KEYFILE "
                     info "Testing mode: adding cryptkey parameter for automated unlock"
                 fi
-                sed -i "s|^GRUB_CMDLINE_LINUX=\"|GRUB_CMDLINE_LINUX=\"cryptdevice=UUID=$uuid:$LUKS_MAPPER_NAME:allow-discards ${cryptkey_param}|" \
+                prepend_grub_cmdline_linux \
+                    "cryptdevice=UUID=$uuid:$LUKS_MAPPER_NAME:allow-discards ${cryptkey_param}" \
                     /mnt/etc/default/grub
                 info "Added cryptdevice parameter for LUKS partition."
             fi
@@ -844,6 +848,9 @@ EOF
 
     # Configure hooks for btrfs
     # Include encrypt hook if LUKS is enabled, btrfs hook if multi-device
+    # No sed verification needed on the four HOOKS= seds below: a missing
+    # HOOKS= line makes mkinitcpio -P fail loudly downstream. (Audited
+    # 2026-04-27 against silent-sed pattern.)
     local num_disks=${#SELECTED_DISKS[@]}
     local luks_enabled="no"
     [[ "$NO_ENCRYPT" != "yes" && -n "$LUKS_PASSPHRASE" ]] && luks_enabled="yes"
diff --git a/installer/lib/common.sh b/installer/lib/common.sh
index 3040799..dfeb245 100644
--- a/installer/lib/common.sh
+++ b/installer/lib/common.sh
@@ -302,3 +302,21 @@ enable_sshd_root_login() {
     grep -q '^PermitRootLogin yes$' "$config_file" \
         || error "PermitRootLogin not set in $config_file (no matching line to replace)"
 }
+
+#############################
+# GRUB Configuration
+#############################
+
+# Prepend a string just inside the GRUB_CMDLINE_LINUX="..." quotes in
+# /etc/default/grub. Errors if the line isn't present in the file.
+# Silently doing nothing here would leave the kernel without the
+# parameter — for cryptdevice= that means the system can't unlock the
+# root partition at boot, so we want a loud failure during install
+# rather than an unbootable system after first reboot.
+prepend_grub_cmdline_linux() {
+    local addition="$1"
+    local config_file="$2"
+    sed -i "s|^GRUB_CMDLINE_LINUX=\"|GRUB_CMDLINE_LINUX=\"${addition}|" "$config_file"
+    grep -qF "GRUB_CMDLINE_LINUX=\"${addition}" "$config_file" \
+        || error "GRUB_CMDLINE_LINUX not modified in $config_file (line missing or pattern unmatched)"
+}