Collapse the two FOR-scope derivations into one

[tobyhede]

Parent

#799. Design record: .scratch/render-execution-unit/spec.md.

What to build

Collapse the two derivations of the FOR-iteration scope into one.

Core builds the step / index / current-item scope twice from the same inputs: once privately inside the compiler for the artifact actor, and once in an exported function whose only production caller was the CLI. Both answer the same question and can drift.

One derivation, called by both the artifact input builder and the entry seam.

Acceptance criteria

• One function derives the FOR scope; the duplicate is deleted rather than left unused.
• Both the artifact resolution path and the entry seam use it.
• A test pins that the two paths see identical scope values for the same state, so a future divergence fails rather than drifts.
• pnpm run verify green; pnpm run test:mutate:changed reports no survivors in the changed ranges.

Blocked by

• Introduce the core entry seam; the execution loop becomes its only caller #819