fix: correct ShapeError in coverage computation for street/grid unit

When using the spatial index, only a subset of geometries (those whose
bounding boxes intersect the buffer) are returned. Assigning their
coverage values directly to self.metadata caused a length mismatch.
Now initialise a full-length list of zeros and fill only the indexed
positions, so all rows in self.metadata always have a coverage value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: koito19960406