Implement local H5 worker service boundary

[anth-volk]

Summary

Implement the PR 8 H5 migration slice by introducing a reusable LocalH5WorkerService for one local-H5 worker chunk and reducing modal_app.worker_script to a CLI/session/JSON adapter.

Scope

• Add a worker-service boundary for executing typed AreaBuildRequest batches against a prepared WorkerSession.
• Preserve the legacy worker JSON summary contract while adding structured results and issues fields.
• Keep --work-items compatibility at the worker-script boundary.
• Add unit coverage for worker service success/failure/validation behavior.
• Add focused worker-script adapter coverage for legacy conversion issues.
• Update build-output engineering docs for the worker chunk execution boundary.

Prerequisite

This work should merge after #973, which it builds on conceptually in the H5 migration sequence.