Nested models (SpatialLocation) don't round-trip through the arrow layer

Problem: the write path is pydantic → dicts → arrow → parquet. flatten_refs collapses a nested model to its id string — fine for references, but SpatialLocation has no id, so it survives as a raw dict while build_arrow_schema maps embedded-model fields to string. A dict then hits a string column: pyarrow errors, or stringifies into an unqueryable text blob. Model → parquet → model loses structure.

Fix options: (a) struct columns in arrow_utils; (b) explicit JSON-column convention; (c) don't nest — #25 flattens spatial and bypasses this. Keep this issue as the general record for any future nested model.

Read the arrow_utils entry points flatten_refs and build_arrow_schema, then trace the pydantic-to-dicts-to-arrow-to-parquet path described in the issue. Compare how nested models and reference models are represented at each step. Done means a SpatialLocation or other nested model can round-trip through parquet without becoming an invalid or unqueryable string.