0005: Feature-switch lifecycle and upgrade support window

Status: accepted (Maintainer, 2026-10-16). Plan: ops/plan-architecture.md.

Implemented: not yet (2026-10-22). No format level exists, and upgrade.yml still rehearses an upgrade from every release since 0.59.1.

Context

Each stage added an EVIDENCE_* switch that, under the deprecation policy, can never be removed before 2.0, and the upgrade rehearsal matrix grows by one version per release.

Decision

• Format-gating switches become a format level shared by the NA and the verifiers; a switch may default to on in a minor release and be removed at the next major, recorded in the deprecation policy.

• Upgrades are rehearsed from the last N minors plus marked long-term releases (N set in upgrade.yml), not from every version ever released.

Consequences

Code paths and CI jobs stop accumulating; operators get a stated support window.