[1.20.1] Worldgen stability fixes: carver crash, flooded caves, legacy preset compatibility

[jmcjm]

What this fixes

Five independent worldgen issues, one commit each:

1. Carver crash below y=0 on extended-height worlds — the carving mask index used the raw block Y (relativeX | relativeZ << 4 | y << 8), which goes negative with MinY below zero and crashes BitSet with IndexOutOfBoundsException as soon as a cave or ravine carves under y=0. The Y is now offset by the world minimum, the same way the ravine height cache a few lines above already does.

2. Legacy 4-argument Dungeon() format — old presets use Dungeon(Frequency,Rarity,MinAltitude,MaxAltitude); the current 3-argument parser silently misreads them, shifting rarity/altitudes by one. Both forms are accepted now (frequency is fixed to 1 either way).

3. Entity() .txt NBT files crashed chunk generation — these files contain text NBT (the old mojangson format), but the code fed them to the binary NbtIo reader, which throws EOFException on any such file — and the handler rethrew it as RuntimeException, killing the whole chunk. Reproducible with Biome Bundle's Ruins/Generic/Cleric.txt. .txt is now parsed with TagParser (SNBT), and a broken file logs an error and skips the entity instead of taking the world down.

4. Out-of-range guards in biome smoothing and CHC lookups — legacy presets carry CustomHeightControl lists sized for 256-high worlds; on extended-height worlds the CHC smoothing indexes past them and crashes. Missing layers are now treated as no height control, and the biome-array lookups in both smoothing loops are bounds/null-checked so a short region from the biome provider degrades to a skipped sample instead of a crash.

5. Everything below the water level was one big flooded cave system — with aquifers disabled, NoiseChunk uses Aquifer.createDisabled(fluidPicker) and every carved or noise-cave block below the biome water level becomes water. The vanilla aquifer noises (values from NoiseRouterData.overworld, 1.20.1) are now wired into the router's aquifer slots and aquifers are enabled in the runtime settings. OTGFluidPicker remains the global picker, so open terrain (oceans, lakes) still floods to the per-biome water level, while caves below it stay dry apart from vanilla-style aquifer pockets and deep lava.

Testing

Manually tested in-game on MC 1.20.1 (Forge 47.4.21, NeoForge 47.1.106, Fabric Loader 0.19.3 + Fabric API 0.92.6+1.20.1): caves below sea level generate dry with occasional aquifer pockets, Biome Bundle no longer crashes on entity ruins, and cave/ravine carving below y=0 no longer crashes. Also smoke-tested as a fabric dev dedicated server (fresh world, level-type=otg:default_preset, no log errors).

[authvin]

This PR conflicts with local fixes I haven't pushed to 1.20.1 yet - will have to wait to merge this until that feature is done