Add local H5 worker sessions

[anth-volk]

Fixes #951

Summary

• Adds local-area H5 worker session setup for source data, weights, geography, validation context, and bootstrap artifacts.
• Passes run-scoped config, artifact paths, and explicit worker scope into local-area H5 workers.
• Consumes valid worker bootstrap artifacts when available, falls back to raw loaders during the dual-path migration, and surfaces worker subprocess stderr on successful runs so bootstrap status is visible in Modal logs.

Verification

• ruff check modal_app/local_area.py tests/unit/test_modal_local_area.py
• uv run --no-sync pytest tests/unit/test_modal_local_area.py -q
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py