ComputePlane: decide when it leaves the legacy classless Talos worker path #5276

Problem / motivation

With #4911, new KubernetesNodes pools get an explicit worker class, and existing pools without a class keep a legacy Talos render. ComputePlane is deliberately left on the legacy path: it creates its child HelmReleases directly, so the class-defaulting webhook never sees them, and its worker allowlist does not expose a class field. It keeps producing classless pools.

That means the legacy path cannot be removed while ComputePlane uses it. The proposal does not say which comes first, or whether the legacy path stays for ComputePlane indefinitely. Open question 13 covers only a possible class choice in the module contract, not the ordering of retirement.

Proposed solution

Decide and record one of two outcomes: either ComputePlane's module contract is extended first (a validated class choice, a version policy, the allowlist and docs updated together) and the legacy path is retired after that, or the legacy path stays for ComputePlane with no end date. I would like a maintainer decision here, and a design-proposal update if the first option is picked.

Scope check

• This affects a single component / package. It touches ComputePlane and the legacy worker path, so a short proposal update may be needed.
• I can describe the expected behavior concretely

Alternatives considered

Writing a fixed class into every ComputePlane group. The proposal rejects this because it would override a class choice added to the module later.

Additional context

Review note: cozystack/community#115 (review) ("ComputePlane and the legacy path"). Implementation: #4911. Related: #4700 (ComputePlane worker image source).