Add optional local origin mode for selected spawn

[ForMooN-0118]

Hi, I made a test implementation for this issue and uploaded both the modified source code and a compiled 1.21.1 jar.

Source branch:
https://github.com/ForMooN-0118/Tellus/tree/local-origin-clean

Test build:
https://github.com/ForMooN-0118/Tellus/releases/tag/local-origin-test-1.21.1

This build adds an optional world-creation UI toggle:

Use selected spawn as local origin

When enabled, the location selected in the Tellus map screen is generated near Minecraft X=0, Z=0 instead of being placed at very large absolute coordinates. I also updated the terrain, biome, water, and OSM-related sampling logic to account for the local coordinate offset.

I tested it with Zhangjiajie as the selected spawn location. The world generated near the origin, and Create Aeronautics / Sable physics contraptions worked normally there. The previous issues I observed at large coordinates, such as contraption misalignment, distorted rope/connection rendering, and airships failing to move correctly, no longer occurred in this test.

I also added a Simplified Chinese localization based on en_us.json.

For transparency, this implementation was AI-assisted. I used AI to help inspect the codebase and draft the changes, but I manually reviewed the modifications, built the project successfully, and tested the result in game.

This is only an unofficial test implementation, but it may be useful as a reference for adding an official local-origin option.

[Goaty1208]

Small issue, Distant Horizons doesn't seem to work with this fix, although it might be just me.

[TimStewartJ]

Hi @ForMooN-0118 — thank you for building and sharing this local-origin prototype, including the in-game Zhangjiajie results.

I independently implemented a broader current-main version before discovering this PR and have now opened draft #52. I explicitly credited and linked #28 there. The draft covers all six currently supported version/loader targets and uses one saved projection across full chunks, Fast Distant Horizons LODs, previews, preloads, OSM, teleports, and cache paths, including the DH alignment gap reported above.

#52 remains a draft pending its final Yosemite runtime smoke test. I do not want to step on your work; I am happy to coordinate or adjust the contribution shape based on your and the maintainers' preference.