H3: Implement CellToBoundary

[lukaskabrt]

Summary

Implement CellToBoundary which returns the geographic polygon boundary of an H3 cell, expressed as SpatialLite geometry types.

Background

Part of #21 – Basic H3 Cell-Mode Support in SpatialLite.

This is the inverse of LatLngToCell. For a given cell index:

1. Decode the index to face-centered IJK coordinates
2. Compute boundary vertices in IJK space (6 for hexagons, 5 for pentagons)
3. Convert each vertex back to lat/lng

Tasks

• Implement H3 index → face-IJK decoding (inverse of the LatLngToCell encoding)
• Implement IJK → lat/lng vertex computation for hexagon and pentagon cells
• Return boundary as IReadOnlyList<Coordinate> (or Polygon) compatible with SpatialLite geometry model
• Handle pentagon cells (5 vertices)
• Handle cells that cross the anti-meridian (180° longitude)

Acceptance Criteria

• Returns correct boundary coordinates for hexagonal cells at all resolutions 0–15
• Pentagon cells return 5 vertices; regular hexagons return 6 vertices
• Anti-meridian crossing is handled correctly
• Return type integrates with the SpatialLite geometry model
• Behavior verified against upstream H3 test vectors