feat: add raspi-arch branch for Arch Linux ARM on Raspberry Pi

[LJLee37]

⚠️ Review-only — do not merge

This is not meant to be merged into raspi. The new raspi-arch branch
has already been pushed directly to raspi-arch (same content as this PR's
head, claude/settingfiles-restructure-raspi-arch-uk2buw-raspi-arch) — there
was no existing raspi-arch branch to open a normal "into" PR against, so
this PR exists purely to give you a reviewable diff. raspi is used as the
base only because it's the closest sibling in scope (both are headless/CLI),
which keeps the diff to just the files that actually differ. Once you're
happy with it, just close this PR without merging; raspi-arch already
stands on its own as a branch.

Summary

Companion to the cleanup PRs on master/arch/macOS/ubuntu/alpine/raspi. Adds the new raspi-arch branch: a headless Arch Linux ARM (ALARM) install for Raspberry Pi, scoped like raspi (CLI-only — no GNOME/Steam like the x86_64 arch branch has).

ALARM's install procedure is fundamentally different from x86 Arch's pacstrap+GRUB flow, so arch's scripts couldn't be reused as-is:

• install-sdcard.sh (replaces install1.sh) — run on a separate Linux host with the SD card/USB drive attached (the Pi has no OS yet at this point). Partitions/formats the media and extracts a prebuilt ALARM rootfs tarball with bsdtar (extended attributes/ACLs can be dropped by plain tar).
• install-firstboot.sh (replaces install2.sh) — run on the Pi after first boot. Adds the ALARM-specific pacman-key --init/--populate archlinuxarm step; drops GRUB/grub-mkconfig entirely (the Pi boots via firmware + config.txt, not GRUB); drops mkinitcpio/initramfs (not needed for a stock boot) and intel-ucode (x86-only). Takes a Pi-model argument to swap in the linux-rpi-16k kernel that a Pi 5 currently needs in place of the generic aarch64 kernel the tarball ships with.
• backup.sh adapted from raspi's: ALARM mounts the boot partition at /boot, not /boot/firmware.
• set.sh: same ~/gitRepos/settingfiles path fix as the other branches.
• .vimrc/.zshrc/init.vim reused verbatim from raspi — unchanged, since shell/editor config is architecture-independent.
• raspi's apt-based install.sh was removed (wrong package manager for ALARM) — flagging per the task's instruction to call out deletions before/while doing them.

Written against a Raspberry Pi 4 Model B (64-bit), using the generic ArchLinuxARM-rpi-aarch64-latest.tar.gz image (ALARM's docs list this as also covering the Pi 3 and 400). The Pi 5 has no dedicated ALARM image as of this writing — install-firstboot.sh 5 swaps in the kernel package the community currently recommends there, but this is inherently more fragile than the Pi 3/4 path. I verified the core steps (tarball URL pattern, bsdtar, pacman-key sequence, fstab entries, no-GRUB boot, Pi 5 kernel caveat) against multiple current third-party guides during this session, since archlinuxarm.org itself blocked automated fetches — please re-check https://archlinuxarm.org for your specific model before a real install, as ALARM's supported devices and exact procedure do change over time.

Hardcoded personal values (carried forward from raspi/arch for consistency, left as-is)

• Default username ljlee, default hostname raspi-arch (both overridable via USERNAME/HOSTNAME_VALUE env vars in install-firstboot.sh — the hostname is a placeholder, not a real personal value, since raspi-arch has no prior hostname to reuse)
• Locale list incl. ko_KR/ja_JP/eo (ported from arch/install2.sh for consistency)
• LAN backup destination 192.168.35.203 (ported from raspi/backup.sh)
• Timezone Asia/Seoul (ported from arch/install1.sh, with the zoneinfo typo already fixed)

Test plan

• bash -n install-sdcard.sh / bash -n install-firstboot.sh — both pass (checked locally)
• .vimrc/.zshrc/init.vim are byte-identical to raspi's (checked locally)
• Actual install against real Pi hardware — not done in this session; please test before relying on this for a real machine

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