feat(build): save build logs to out/ alongside the ISO

Before this commit, mkarchiso's verbose output streamed to the terminal and disappeared. The cleanup trap removed work/ after each build, taking any captured output with it. Failed builds left no log to compare against past failures — including the recurring archzfs cache corruption that hits ~2 of 3 builds.

I added a tee on the mkarchiso pipeline that writes to a log file in out/. The log starts as out/build-YYYY-MM-DD-HHMM.log, pre-created and chowned to SUDO_USER. It stays user-readable even when a failed build leaves it under that name. On success, the script renames it to match the ISO's basename so finding the log for a given ISO is a one-step lookup.

set -o pipefail was already on at the top of build.sh, so mkarchiso failures still propagate through the pipe and abort the script the way they used to. No exit-code masking.

I verified the change end-to-end against a real build that hit archzfs cache corruption (log captured, kept as build-2026-05-19-0325.log, user-readable) followed by a clean retry (log renamed to archangel-2026-05-19-vmlinuz-6.18.29-lts-x86_64.log, paired with the ISO).

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