test: expand bats coverage across installer modules

Added unit tests for `disk.sh`, `btrfs.sh`, the archangel monolith's `gather_input` unattended branch, and filled gap cases in `config.sh`. The suite grew from 71 to 110 tests.

`installer/lib/disk.sh` was completely uncovered. New `tests/unit/test_disk.bats` covers the four pure partition-path helpers (`get_efi_partition`, `get_root_partition`, `get_efi_partitions`, `get_root_partitions`) across SATA, virtio, and NVMe inputs, mixed arrays, and the empty-input behavior. Side-effecting functions in the same file (sgdisk, mkfs.fat, partprobe, and fzf wrappers) stay deliberately VM-tested.

`installer/lib/btrfs.sh` had no bats coverage. New `tests/unit/test_btrfs.bats` covers `get_luks_devices`, the only pure helper in the file. It pins the asymmetric naming convention where the first device gets the bare `LUKS_MAPPER_NAME` and subsequent devices append the index.

The archangel monolith was un-source-able for tests because its top-level code created a /tmp log file and redirected stdout via `exec > >(tee...)`, plus called `main "$@"` unconditionally at the bottom. I extracted the logging setup into an `init_logging` function called from `main`, and wrapped the main call in a `[[ "${BASH_SOURCE[0]}" == "${0}" ]]` guard. Sourcing the script now loads function definitions silently, with no log file and no banner. Running it directly works exactly as before. Verified both paths.

That refactor unlocks `tests/unit/test_archangel.bats`, which covers `gather_input` in unattended mode. Required-field validation for HOSTNAME, TIMEZONE, ROOT_PASSWORD, and DISKS. Optional-field defaulting (FILESYSTEM to zfs, LOCALE to en_US.UTF-8, KEYMAP to us, ENABLE_SSH to yes). Filesystem-specific encryption checks (ZFS_PASSPHRASE required when not NO_ENCRYPT, same for LUKS_PASSPHRASE on Btrfs). Filesystem validity. RAID_LEVEL defaulting for multi-disk installs. The interactive branch stays out of scope per the testing-strategy policy.

`tests/unit/test_config.bats` got five gap tests: `check_config` when CONFIG_FILE is set, `validate_config` against a non-block-device entry (e.g. /dev/null) and a missing path, and `parse_args` accepting `--color` and `--config-file` together in either order.

`testing-strategy.org` got an expanded "What bats does NOT cover" section. The doc previously named six tools (mkfs, cryptsetup, zpool create, pacstrap, arch-chroot, grub-install). The new list adds sgdisk, partprobe, blkid, mkfs.fat, mkfs.btrfs, snapper, efibootmgr, mount, umount, findmnt, mountpoint, and fzf. It also names the conditions (root needed, real /dev or /sys state) that make a function VM-only. The coverage table at the top now lists the three new test files.

No behavior change in production code. The init_logging extraction preserves the existing log path and banner format byte-for-byte.

1 files changed, 21 insertions, 10 deletions

diff --git a/installer/archangel b/installer/archangel
index a40710b..b2fa2c0 100755
--- a/installer/archangel
+++ b/installer/archangel
@@ -61,16 +61,21 @@ RAID_LEVEL=""          # "", "mirror", "raidz1", "raidz2", "raidz3"
 ENABLE_SSH="yes"       # Enable SSH with root login (default yes for headless)
 NO_ENCRYPT="no"        # Skip ZFS encryption (for testing only)
 
-# Logging
-LOGFILE="/tmp/archangel-$(date +'%Y-%m-%d-%H-%M-%S').log"
-exec > >(tee -a "$LOGFILE") 2>&1
+# Logging — initialized by init_logging() at install time only. Sourcing
+# this script (e.g. from a bats test) loads the function definitions
+# without redirecting stdout or creating a log file in /tmp.
+LOGFILE=""
 
-# Log header with timestamp
-echo ""
-echo "================================================================================"
-echo "archangel started @ $(date +'%Y-%m-%d %H:%M:%S')"
-echo "================================================================================"
-echo ""
+init_logging() {
+    LOGFILE="/tmp/archangel-$(date +'%Y-%m-%d-%H-%M-%S').log"
+    exec > >(tee -a "$LOGFILE") 2>&1
+
+    echo ""
+    echo "================================================================================"
+    echo "archangel started @ $(date +'%Y-%m-%d %H:%M:%S')"
+    echo "================================================================================"
+    echo ""
+}
 
 # Output functions now in lib/common.sh
 # Config functions now in lib/config.sh
@@ -1385,6 +1390,7 @@ print_btrfs_summary() {
 #############################
 
 main() {
+    init_logging
     parse_args "$@"
     preflight_checks
     check_config
@@ -1490,4 +1496,9 @@ install_btrfs() {
 
 trap 'error "Installation interrupted!"' INT TERM
 
-main "$@"
+# Only invoke main when archangel is executed directly. When sourced
+# (e.g. from a bats test) the function definitions load but main does
+# not run, so tests can exercise individual helpers like gather_input.
+if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+    main "$@"
+fi