Add linux/arm64 preview images and certify the install path

[Maneek21]

Current state

Refreshed September 24, 2026 against master at 1427f66.

Official release builds and release manifests still target linux/amd64. ARM64 is new supported-platform work, not a completed feature or a regression.

Evidence: release workflow, manifest generator, limitations.

Remaining work and acceptance evidence

When ARM64 demand and an implementation owner are established, publish and certify an ARM64 release path.

• Publish a multi-architecture image or a documented separate linux/arm64 image.
• Run installation, doctor, and browser smoke on ARM64 hardware or clearly identified emulation.
• Record tested platform/environment and exact image identity.
• Keep signatures, provenance, manifests, and operator instructions accurate for every supported image.
• Verify the existing amd64 path remains functional.

Ownership and priority

Unassigned; Later. Excluded from Next preview hardening. Revisit when a concrete ARM64 installation need and a maintainer are identified; no delivery date is promised.