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Estructuras con Mobs 1.5.4 - Forge 1.21.1

Automated QA baseline for Minecraft Forge 1.21.1.

## Quality assurance

- Added fast automated tests for all 42 feature registrations, spawner items, configured features, placed features, item models and 32 x 32 textures.
- Added server GameTests for Forge registries, persistent spawner data, player-aware loot, safe manual height limits and deterministic chunk-clipped mega structures.
- The automated suite now rejects missing or mismatched worldgen resources instead of reporting `NO-SOURCE`.
- Established an auditable local Git baseline while excluding build output, test worlds, logs and release binaries.

## Requirements

- Minecraft 1.21.1
- Forge 52.1.0 or newer within the Forge 52 branch
- Java 21+
- Required on client and server
- No external dependencies