Add H3 POI attribution and passthrough aggregation

[paco-barreras]

Why

Grid-Based stop tables can contain H3 containment areas without coordinates, but visit attribution previously required point geometry. Direct stop-table APIs also could not forward custom passthrough aggregation, forcing callers back to label APIs and external summarization.

What changed

• Add an H3 branch to poi_map that expands all POI geometries to intersecting cells at the input resolution, computes bounded grid distances with H3ronPy's native Arrow kernel, and returns aligned POI identifiers using existing poi_map conventions.
• Resolve repeated containment areas once, preserve missing and unmatched values, reproject POIs to WGS84, reject mixed input resolutions, and break distance ties by POI table order.
• Expose passthrough_agg on all direct and per-user stop-table APIs and forward it only to stop summarization, including Grid-Based.
• Preserve first-value passthrough behavior when no custom aggregator is supplied and preserve empty stop-table schemas.

Validation

• 22 passed in test_visit_location_detection.py
• 25 passed, 1 xfailed in test_stop_detection_utils.py
• 11 passed in the focused direct/per-user/Grid-Based stop-detection selection
• No VS Code diagnostics in touched Python files

Closes #433
Closes #434

This pull request includes code written with the assistance of AI. The code has not yet been reviewed by a human (remove this disclosure after human review).