greenR 0.0.1.8

• Calculate the street green index with local metric sf distances, removing the runtime dependency on the DuckDB spatial extension. Empty optional green area and tree layers now contribute zero instead of causing an error.
• Fetch the selected OpenStreetMap features in one Overpass request, support configurable endpoints and caching, and validate geographic bounding boxes.
• Correct several existing mapping, accessibility, and spatial-processing edge cases; add offline regression tests and update documentation.