refactor: extract pure RAID logic to lib/raid.sh with bats coverage

Peel the testable pieces of get_raid_level() out of the 1600-line
installer monolith into installer/lib/raid.sh:

- raid_valid_levels_for_count(count) — replaces the inline option-list
  builder in get_raid_level()
- raid_is_valid(level, count) — useful for unattended-config validation
- raid_usable_bytes(level, count, smallest, total) — usable-space math
- raid_fault_tolerance(level, count) — max tolerable disk failures

archangel now sources lib/raid.sh and uses raid_valid_levels_for_count
for the fzf option list. Fzf preview subshell still inlines its own
usable-bytes arithmetic (calling exported lib functions across preview
subshells is fragile; left for a later pass).

30 bats tests in tests/unit/test_raid.bats cover the full enumeration
table, every valid/invalid level-vs-count combo from 2 to 5 disks,
mixed-size mirror, and unknown-level error paths. make test: 53/53.

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