Fix cave biomes leaking into non-overworld dimensions' biome sources

[UwUOcha]

Fix cave biomes leaking into non-overworld dimensions' biome sources

The bug

MultiNoiseBiomeSourceMixin#possibleBiomes unconditionally adds nomansland:caves and nomansland:cave_depths to the possible-biome set of every MultiNoiseBiomeSource, regardless of which dimension the source belongs to — including the vanilla nether source inside the minecraft:the_nether level stem and modded dimensions like deeperdarker:otherside.

In a vanilla nether this is invisible (the biomes are never actually picked), but it breaks badly together with BetterNether / WorldWeaver (wover): wover replaces the nether biome source with its own WoverNetherBiomeSource and merges the biomes of the source it replaces. Its MergeableBiomeSource logic force-adds every merged biome that isn't already in #minecraft:is_nether into that tag and into its nether biome picker. As a result the two overworld cave biomes actually generate in the nether as full-fledged nether biomes.

Since those biomes are in #minecraft:has_structure/mineshaft, #minecraft:has_structure/trial_chambers, #minecraft:has_structure/ruined_portal_standard and #c:is_cave, this drags mineshafts, trial chambers, ruined portals and any third-party structure sets targeting cave/underground tags into the nether. It also feeds overworld mob-spawn lists into nether chunk generation, which for some players escalates into worldgen-thread stalls/freezes (observed with Alex's Caves' NaturalSpawner chunk-generation spawning, e.g. striders with jockeys mounting during worldgen).

Log evidence from a pack with BetterNether (wover's world-dimensions report at world load), before the fix:

 - minecraft:the_nether:
     WoVer - Nether BiomeSource (50409aa6)
         biomes     = 29
         namespaces = betternether(22), minecraft(5), nomansland(2)
...
[wover-generator/]: Merged 2 biomes to WoVer - Nether BiomeSource (50409aa6) in 599.6 μs

The same two biomes also leak into deeperdarker:otherside:

     MultiNoiseBiomeSource (ac6381e)
         biomes     = 6
         namespaces = nomansland(2), deeperdarker(4)

This cannot be worked around by users: the mixin has no config gate, so disabling the cave biomes in the config does not stop the leak.

The fix

Gate the injection on the biome source actually belonging to an overworld dimension, using the dimension type Biolith already tracks per biome source (InterfaceBiomeSource#biolith$getDimensionType) — the same check Biolith itself uses when deciding which placement's entries to inject. The cave biomes are only ever placed in the overworld (DimensionBiomePlacementMixin guards on OverworldBiomePlacement), so reporting them as possible biomes of other dimensions' sources is never correct.

Testing

Tested on 1.21.1 / NeoForge with a ~230-mod pack including BetterNether 21.0.20, Deeper and Darker, TerraBlender and Biolith 3.0.12. After the fix:

• Nether source is clean: biomes = 27, namespaces = betternether(22), minecraft(5); the Merged 2 biomes line is gone.
• deeperdarker:otherside is clean: namespaces = deeperdarker(4).
• Overworld is unaffected: namespaces = minecraft(53), spawn(10), nomansland(22), yungscavebiomes(2) — same as before the fix, cave biomes still generate and decorate normally.
• No mixin errors in the log.

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