[zachspartofaday]

Add a prepared SteamOS ARM64 userspace experiment targeting a desktop and terminal. The supplied SteamFrame image contains ARM64 programs, but its default VR/gamescope session and service assumptions do not fit LinuxRunner. This change installs a verified userspace archive, uses Plasma X11, and offers the existing Metal/virgl groundwork through a compatible isolated Mesa overlay.

The installer checks the exact source/archive identity, size and free space, supports cooperative cancellation and normal account setup, and publishes a new environment. Archive import preserves validated PAX UTF-8 names in the C locale without changing process/thread locale, bounds and verifies its private copy, and cleans failed staging/resources. Generic imports retain their 512 MiB bound; only the exact prepared manifest admits the larger SteamOS archive.

SteamOS package operations remain inspection-only. Preserving per-account defaults disable unavailable service applets and Steam autostart, select Fusion after a Breeze crash, use grayscale font antialiasing for scaled output, and keep the reversible tags-worker workaround nonfatal. The optional GPU path checks its isolated Mesa payload and startup pixels before Xorg/Qt use; failed prerequisites select the software desktop. KWin effects stay disabled. Linux networking uses iPadOS sockets and DNS, not Linux Wi-Fi drivers.

This consolidates clean-reviewed, merged fork PRs #5, #8 and #10 into one commit directly on main. The complete accelerated experience also requires #19 (virtual DRM/host GPU) and #20 (desktop input/capture/presentation). Storage stays separate in #18. We recommend reviewing those shared prerequisites first; they are not duplicated in this diff.

The SteamOS image is supplied separately and is not included. The related 4.77 MB guest Mesa overlay retains pinned source identity, build/packaging instructions, license inventory, probe source and hashes. The packager recovers its preferred probe source from the verified archive if the standalone source from #19 is absent. Normal app identity/entitlements remain unchanged unless an operator explicitly selects the separate experiment build overrides.

Validation:

• Grouped offline checks pass: 10 exporter/overlay contracts, 6 preserving font-default tests, 13 desktop-configuration tests, Swift manifest identity/hash/size/overflow contracts, Python compilation and the 97-resource manifest.
• Release iOS Simulator app and UI test build-for-testing passes with JIT and native VIRGL disabled. This is compile-only, not a physical-device run.
• The unchanged importer had already passed a freshly compiled macOS fixture using pinned libarchive: Unicode files/symlinks, locale/source preservation, invalid UTF-8/truncated gzip rejection, source/hash mismatch, symlink rejection and cancellation.
• Composing this branch with the other grouped submissions reproduces the merged fork’s product/test/evidence tree byte-for-byte. Historical physical integration evidence establishes Plasma/Konsole launch and aarch64, user-reported Magic Keyboard operation, and DNS/HTTPS; it is not exact-branch requalification.

Steam client, gamescope, Vulkan, games, audio/system services, sustained performance and complete application compatibility remain outside the milestone. See docs/STEAMOS_EXPERIMENT.md, docs/ROOT_IMPORT.md and THIRD_PARTY_NOTICES.md for preparation, rollback, source availability and limits.