fix(installer): RAID validation, set -e fix, drop dead shadow branch

Two installer cleanups from the todo backlog.

validate_config now rejects a RAID_LEVEL the selected disk count can't support, guarding the unattended path (the interactive path already constrains the choice). While adding it I found a latent bug: the error loop's ((errors++)) returned 0 on the first error and tripped set -e in the monolith's `[[ UNATTENDED == true ]] && validate_config` call, aborting after one warning instead of listing every problem. Switched to pre-increment so the count accumulates as designed. Added four bats cases, including one that runs validate_config under set -e outside bats' run shield.

build.sh dropped the dead shadow-file rebuild else-branch. The profile is always copied fresh from releng (which ships /etc/shadow), so the branch never ran, and its hardcoded account list had drifted from what releng provides. Replaced with an assertion that fails the build loudly if the file is ever missing.

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