TerrainNode boundaries across 180° longitude don't wrap

Creating a terrain node with bounds crossing 180° longitude fails. For example, New Zealand can't be rendered in a single TerrainNode.

Example:`TerrainNode(minLat: -44.0, maxLat: -36.0, minLon: 169.0, maxLon: -179.0)`.

I've added some simple checks in #59 to avoid crashes, but not a fix.

Start at the TerrainNode entry point and reproduce the example bounds spanning 180° longitude: minLon 169.0 and maxLon -179.0. Verify that the node can be created without crashing and renders the full region, including New Zealand, as the completion criteria.

Written by the indexing model from the issue text.