Formalize multi-system support policy and evidence ladder

[CMCDragonkai]

Problem

Repository policy is intentionally single-system today (x86_64-linux), but support-expansion criteria and evidence requirements are not yet formalized.

Intent

Prevent accidental or aspirational system-surface growth by requiring explicit policy and check evidence for each support claim.

Scope

• Define support-level ladder (declared, evaluates, builds, supported).
• Define per-system evidence/check requirements.
• Define process for adding new systems.

Non-goals

• No immediate requirement to add more systems.
• No broad matrix expansion without accepted policy.

Tasks

1. Write multi-system support policy.
2. Link policy from README.
3. Add checks/guardrails for declared system surfaces.

Acceptance criteria

• Support levels are documented and unambiguous.
• System additions require defined evidence.
• Check behavior protects against accidental surface drift.

[linear]

INF-152