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, 56 insertions, 0 deletions

diff --git a/tests/unit/test_btrfs.bats b/tests/unit/test_btrfs.bats
new file mode 100644
index 0000000..890bba2
--- /dev/null
+++ b/tests/unit/test_btrfs.bats
@@ -0,0 +1,56 @@
+#!/usr/bin/env bats
+# Unit tests for installer/lib/btrfs.sh
+#
+# Coverage scope: pure helpers only. Most of btrfs.sh wraps cryptsetup,
+# mkfs.btrfs, snapper, grub-install, and arch-chroot — all deliberately
+# VM-tested per the project's testing-strategy.org policy. Only
+# get_luks_devices is covered here.
+
+setup() {
+    # shellcheck disable=SC1091
+    source "${BATS_TEST_DIRNAME}/../../installer/lib/common.sh"
+    # shellcheck disable=SC1091
+    source "${BATS_TEST_DIRNAME}/../../installer/lib/btrfs.sh"
+}
+
+#############################
+# get_luks_devices
+#############################
+# Asymmetric naming: index 0 uses the bare LUKS_MAPPER_NAME (no
+# suffix), subsequent indices append the index. Tests pin both the
+# bare-first behavior and the suffix-on-rest behavior.
+
+@test "get_luks_devices: count=1 emits the bare-named mapper device" {
+    LUKS_MAPPER_NAME="cryptroot"
+    run get_luks_devices 1
+    [ "$status" -eq 0 ]
+    [ "$output" = "/dev/mapper/cryptroot" ]
+}
+
+@test "get_luks_devices: count=3 emits bare name + suffixed entries" {
+    LUKS_MAPPER_NAME="cryptroot"
+    run get_luks_devices 3
+    [ "$status" -eq 0 ]
+    [ "$output" = "/dev/mapper/cryptroot /dev/mapper/cryptroot1 /dev/mapper/cryptroot2" ]
+}
+
+@test "get_luks_devices: count=0 emits empty output" {
+    LUKS_MAPPER_NAME="cryptroot"
+    run get_luks_devices 0
+    [ "$status" -eq 0 ]
+    [ -z "$output" ]
+}
+
+@test "get_luks_devices: count=5 emits five entries with correct suffix progression" {
+    LUKS_MAPPER_NAME="cryptroot"
+    run get_luks_devices 5
+    [ "$status" -eq 0 ]
+    [ "$output" = "/dev/mapper/cryptroot /dev/mapper/cryptroot1 /dev/mapper/cryptroot2 /dev/mapper/cryptroot3 /dev/mapper/cryptroot4" ]
+}
+
+@test "get_luks_devices: non-numeric count is treated as zero (no crash)" {
+    LUKS_MAPPER_NAME="cryptroot"
+    run get_luks_devices abc
+    [ "$status" -eq 0 ]
+    [ -z "$output" ]
+}