H3: Implement CellToParent and CellToChildren

[lukaskabrt]

Summary

Implement the hierarchical operations CellToParent and CellToChildren which navigate up and down the H3 cell hierarchy by resolution.

Background

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

H3 encodes hierarchy directly in the index bit structure:

• CellToParent: replace child digit bits below the target resolution with the padding value (7)
• CellToChildren: enumerate all valid digit combinations for the added resolution levels

Each regular hexagon has exactly 7 children per resolution step. Pentagon cells have 5 children per step due to a missing digit direction in their structure.

Tasks

• Implement CellToParent(H3Cell cell, int parentResolution) → H3Cell
  • Validate that parentResolution is coarser (lower number) than the cell's current resolution
• Implement CellToChildren(H3Cell cell, int childResolution) → IEnumerable<H3Cell>
  • Validate that childResolution is finer (higher number) than the cell's current resolution
  • Enumerate all valid digit combinations, skipping digit 1 for pentagon cells

Acceptance Criteria

• CellToParent returns the correct parent for all resolution combinations
• CellToChildren returns exactly 7 children for hexagons and 5 for pentagons per resolution step
• Total child count for multi-step traversal: 7^(childRes − cellRes) for hexagons
• ArgumentOutOfRangeException thrown for invalid resolution arguments
• Behavior verified against upstream H3 test vectors