Support nested item model references

[mneuhaus]

Summary

This supports nested item model references during extraction. The change is needed for the Minecraft 26.1.2 asset update chain, where item model references may be nested instead of appearing only at the top level.

Testing

Validated as part of the local 26.1.2 asset generation and viewer compatibility pass.

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 #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 prismarine-physics#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.