SkyLands A Universal - Large Islands + Structures - No Micro Fragments

Purpose:
- Keep the existing large-islands-only, sparse themed groups, HeightPlus100 / MinPlus50 / Densityx3 tuning.
- Restore real SkyLands jigsaw structures in the same vanilla/modded biome targets used by this datapack.
- Keep all raw_generation scattered island features disabled.
- Keep medium/small standalone island features and fallen-island fragments disabled.
- Remove isolated 1-block cloud fragments embedded in the retained large-island templates.

Restored structures:
- Common/default: floating dungeon, floating tower, floating village, sky arena.
- Cherry: cherry floating dungeon, cherry tower.
- Arid: arid sky temple, desert sky arena.
- Glaciated: ice castle, ice dungeon, ice sky arena.
- Lavic: lava railway, lava tower, lava sky arena.

Not restored intentionally:
- raw_generation island features (fis/fim/fir/dfis/fiis/fils/etc.).
- fim_2 medium standalone island.
- fallen_is_01 / fallen_is_02 fragments.
- small spike/debris biome features that can clutter the sky.
- lava huge-spike and ground-decoration structure sets.

Placement:
- Structure sets are made much sparser than the original mod so they do not overwhelm the tuned island density.
- Common structures use #flying_stuff:large_island_common_spawnable, matching the current fallback behavior for modded Overworld biomes.
- Special structures use the corresponding arid/cold/cherry/lavic biome tags.
- SkyLands has no dedicated swamp building set; swamp biomes can receive the sparse common/default structures through the common fallback tag.

Important:
- Only newly generated chunks are affected.
- Structures are independent floating jigsaw structures; they are not guaranteed to be physically attached to every large island.
- Use only one SkyLands tuning datapack at a time.
