Add three methods that create NEW HeightMap objects:
- threshold(range): preserve original values where in range, 0.0 elsewhere
- threshold_binary(range, value=1.0): set uniform value where in range
- inverse(): return (1.0 - value) for each cell
These operations are immutable - they preserve the original HeightMap.
Useful for masking operations with Grid.apply_threshold/apply_ranges.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
