Build images from the aarch64-apple runtime layout

[maralcbr]

Summary

omacom/omarchy#14431 merged into quattro (e1b0e5e9b) with the platform rename (x86, aarch64, aarch64-apple). The image builder could not build against it: it looked only for omarchy-apple-silicon.packages / omarchy-apple.packages, and it fell back to the plain aarch64 pacman template (no asahi-alarm) because it skipped default/pacman/aarch64-apple/.

• build-mac-image, candidate_set.py and inspection.py take the Apple list as omarchy-aarch64-apple.packages, then 5397950a2's omarchy-apple-silicon.packages, then the oldest omarchy-apple.packages (never a link, as before).
• The installed pacman.conf comes from the runtime's default/pacman/aarch64-apple/ first, then omarchy-mac's /usr/share/omarchy-mac/pacman, apple-silicon or aarch64 as before. The mirror list comes from beside the aarch64-apple template (Omarchy's HTTPS Arch Linux ARM mirror), else aarch64/, else mirrorlist-aarch64. Inspection checks the same choice. Older runtimes resolve exactly as before.
• The image-target manifest still says platform=apple-silicon. e1b0e5e9b reads it as aarch64-apple; 5397950a2, which released images and packages still run, rejects aarch64-apple in omarchy-hw-platform and install/helpers/image-target.sh. A comment next to the write says so.
• Test fixtures now come in two shapes, one for each runtime (e1b0e5e9b and 5397950a2). Both must verify as a candidate set and pass inspection, and neither may install the plain aarch64 template. The shell tests now also cover how the Apple list and the installed pacman config are chosen.
• Unchanged on purpose: the platform= field in IMAGE and PROVENANCE, omarchy_target: apple-silicon-full-os, and the aarch64-apple-silicon-mac zip names. These are contracts with the installer and catalog, not runtime platform names.

Behaviour change for review: on a runtime that ships default/pacman/aarch64-apple, the runtime's template now wins over omarchy-mac's /usr/share/omarchy-mac/pacman. That matches what the runtime installs itself (install/helpers/pacman.sh). Current omarchy-mac no longer ships that directory.

Test plan

• image-builder/test/all in an Arch Linux ARM container: passes at each commit
• The new tests fail against main's builder sources (Apple list, template choice, e1b0e5e9b layout)
• Dry run against real runtime trees, packing each tree as the candidate omarchy archive. e1b0e5e9b resolves to omarchy-aarch64-apple.packages, aarch64-apple/pacman-edge.conf (with asahi-alarm) and the HTTPS mirror list. 5397950a2 resolves to omarchy-apple-silicon.packages, apple-silicon/pacman-edge.conf and aarch64/mirrorlist-edge, the same as before.
• test/all (portable) on macOS: passes
• Lab image build against an e1b0e5e9b candidate set. This needs Follow #14431's platform rename to aarch64-apple omarchy-mac-pkgs#24 or Remove older Asahi-based installs and return leftover free space #26 first: omarchy-mac-boot's require_apple_silicon still compares omarchy-hw-platform with apple-silicon.