Add 26.1 physics feature flags

[mneuhaus]

Summary

Adds 26.1 feature flags needed by prismarine-physics for the Minecraft 26.1.x compatibility path.

This is part of the 26.1.2 compatibility chain used by Mineflayer external tests.

Testing

Validated locally with:

• npm test
• 20 passing

Evidence summary: https://gist.github.com/mneuhaus/decaae9cd8767a651272d31cdf97ac60

Related PRs

Part of the Minecraft 26.1.2 update chain:

1. Support nested item model references minecraft-jar-extractor#67 - nested item model extraction support
2. Add Minecraft 26.1.2 assets minecraft-assets#48 - generated 26.1.2 assets
3. Add Minecraft 26.1.2 assets wrapper node-minecraft-assets#52 - assets wrapper update
4. Add Minecraft 26.1.2 data minecraft-data#1186 - 26.1.2 protocol/data plus generated block/item data
5. Update wrapper data for Minecraft 26.1.2 node-minecraft-data#455 - wrapper update for the generated data
6. Add Minecraft 26.1.2 protocol support node-minecraft-protocol#1487 - 26.1.2 protocol support/tests
7. Support Minecraft 26.1.2 chunk fluid counts prismarine-chunk#326 - 26.1.2 chunk fluid count support
8. Add 26.1 physics feature flags #134 - 26.1 physics feature flags
9. Update Mineflayer for Minecraft 26.1.2 mineflayer#3902 - full Mineflayer external-suite compatibility
10. Support Minecraft 26.1.2 in prismarine-viewer prismarine-viewer#480 - 26.1.2 viewer support

The dependent PRs are opened as drafts so they can be reviewed/merged in order.

Implementation note

Prepared with assistance from GPT-5.5 and validated locally with the checks listed above.