refactor: decompose install_btrfs into named orchestration stages

Pull the single-vs-multi-disk and LUKS-vs-no-encryption branching out
of install_btrfs() into five helpers in lib/btrfs.sh:

- btrfs_open_encryption         — LUKS open + fill devices array
- btrfs_make_filesystem         — create_btrfs_volume dispatch
- btrfs_configure_luks_target   — in-chroot LUKS config
- btrfs_install_grub            — GRUB primary + multi-disk mirror
- btrfs_close_encryption        — LUKS close (cleanup)

Helpers use namerefs (local -n) to take the caller's arrays as locals
instead of promoting them to globals. install_btrfs() drops from ~99
lines of nested if-then-else to a ~45-line flat sequence of named
stages — matching the style of install_zfs().

Behavior preserved — this is pure code movement, no new disk/LUKS
operations. No unit tests added for the new helpers: they all wrap
real LUKS/mkfs.btrfs calls that need block devices and root; VM
integration tests in scripts/test-install.sh remain the source of
truth. .shellcheckrc: disable SC2178 (nameref array heuristic) and
SC2153 (globals from sourced files) — both recurring false positives.

make test: 65/65. shellcheck clean.

0 files changed, 0 insertions, 0 deletions