refactor: extract validate_encryption_passphrase from gather_input

gather_input's unattended branch had two parallel if-blocks, one for ZFS and one for Btrfs, each doing the same encryption-passphrase empty check against a filesystem-specific variable (ZFS_PASSPHRASE or LUKS_PASSPHRASE). The two blocks shared the condition surface and error template. Only the variable name differed.

I lifted the check into validate_encryption_passphrase in lib/config.sh next to validate_filesystem. The helper takes the variable name and uses indirect expansion (${!var_name}) so one function covers both filesystems. gather_input now dispatches via if/elif on FILESYSTEM and calls the helper with the right variable, collapsing 14 lines to 6.

The original tests in test_archangel.bats (gather_input errors when ZFS without ZFS_PASSPHRASE / when Btrfs without LUKS_PASSPHRASE / accepts ZFS with NO_ENCRYPT=yes) still pass, exercising the helper through the dispatch. Added 4 direct unit tests in test_config.bats covering the four cases: NO_ENCRYPT=yes passes regardless, NO_ENCRYPT=no with empty fails, NO_ENCRYPT=no with value passes, and the error message names the offending variable. Bats: 177 → 181.

No behavior change. The helper preserves the original error message format and exit conditions.

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