Type model file values as JSON

[adamtheturtle]

Summary

• use the existing recursive JSON type throughout the model-file decoding pipeline
• retain object only at the raw decoder and shape-validation boundaries
• avoid a broadly typed intermediate by validating root arrays and objects in their respective branches

Validation

• uv run --group=dev prek run --all-files --hook-stage pre-commit
• uv run --group=dev prek run --all-files --hook-stage pre-push
• uv run --group=dev pytest -q (157 passed)