Install published ARM core packages by default

[scottjones]

Normal Mac installations currently rebuild four core packages locally even though the ARM repository publishes them. This changes the default to download the core packages from omarchy-aarch64, validate the complete release, and run setup and package lists from the installed release.

omarchy and omarchy-settings must have the same full version, including pkgrel. The downloaded archives must include the required Snapper dependency, packaged Mac installer protocol, Apple boot configuration, and applicable keyboard brightness service. Missing or mismatched releases fail with an explanation and never silently trigger a source build.

Development installs explicitly use --from-source; custom repositories or refs require it, and guided setup preserves the choice across resume and reboot. ARM installation CI uses this option to test the proposed checkout.

Rollout prerequisite

Keep this PR in draft until a compatible core package pair has been built and validated on ARM. The currently published 4.0.2-2 release lacks required contents and is intentionally rejected. The packaged helper introduced here must be included in the candidate release before enabling this bootstrap for normal users. This PR does not publish a release.

Validation

• Archive fixtures exercise matching releases, rejected metadata and missing payloads, source opt-in, installed-release handoff, and resume persistence.
• Read-only inspection of the actual published 4.0.2-2 archives confirms the default path rejects the missing Snapper dependency before installing core packages.
• CLI suite and 266 of 267 shell test files pass in the sandbox. The remaining network QR fixture passes when rerun with route access; the sandbox denies its netlink query.
• All 142 guided-setup checks, command metadata, syntax checks, and whitespace checks pass. A real installation of a newly built prebuilt release remains a rollout prerequisite.