Stage 1 command runner and substep status boundary

[anth-volk]

Context

Stage 1 PR 3 introduces an explicit command runner and substep/status boundary around the existing dataset-build execution flow.

Scope

• Add structured command, result, status, and coordinator helpers for Stage 1.
• Keep current script modules, Makefile commands, execution order, checkpoint behavior, and log streaming.
• Wrap Modal data-build command execution with explicit substep results and status/error records.
• Add focused unit coverage for command construction, structured failures, substep timing, skip states, and artifact collection.

Validation

Targeted unit, docs, quality, and lint checks will be reported on the linked draft PR.