fix: add full Minecraft 1.18+ support for negative Y coordinates

[keithworrell]

Fixes several critical bugs preventing proper rendering of Minecraft 1.18+ worlds with negative Y coordinates (Y=-64 to Y=320).

Issues Fixed

1. Chunk sections not rendering on initial load Files: viewer/lib/worldrenderer.js (addColumn, removeColumn) Problem: Y loop hardcoded to 0-256, ignoring negative Y sections Solution: Changed range to -64 to 320 (full 1.18+ world height)

2. Section index calculation errors

File: viewer/lib/worker.js
Problem: Math.floor(y / 16) produces negative array indices for Y < 0 Solution: Use Math.floor((y - chunk.minY) / 16) to account for minY offset

3. Unknown entity crashes

File: viewer/lib/entity/Entity.js
Problem: Throws error when encountering unknown entity types Solution: Log warning and create placeholder mesh instead

4. Null block handling

File: viewer/lib/models.js
Problem: Crashes when chunk not fully loaded
Solution: Skip null blocks gracefully

Testing

Tested on Minecraft 1.21.1 (protocol 767) with:

• Flat creative world at Y=-60
• Underground structures at Y < 0
• Chunk loading/unloading
• Browser refresh persistence

Before/After

Before: Chunks at negative Y don't render until blocks are modified After: All chunks render immediately on initial load

Fixes rendering for underground bases, caves, and flat worlds introduced in Minecraft 1.18 (Nov 2021).