Isekai API

No Java API breaks. The one addition is a default method, so SurfaceAnchor variants written against 1.x / 2.0.0 / 2.1.0 compile and run unchanged. The datapack schema is untouched; what changes is where ore lands in a column that holds more than one body.

Fixed — datapack behaviour

• isekai_api:column_relative reaches every body in a column, not just the topmost one. A column of floating terrain can hold several islands stacked above one another, and both default anchors resolved against the highest of them by construction — world_surface reads the heightmap, and world_floor scans down from whatever that found. An island with anything floating above it therefore received no ore at all. It now emits one position per body, at the same normalized depth, so an island's ore no longer depends on what happens to float above it. Measured on Sky World: andesite, granite, diorite, coal, iron and copper were all exactly 0 through the lower island band, in a region where 29 of 29 lower-island chunks also carried an upper island.
• A body too thin for the band is skipped on its own instead of ending the column, so a thin island no longer hides the ones below it.
• A column holding one body places exactly what it placed before, at the same Y, having drawn the same amount from the random source — the depth is drawn once per placement however many bodies the column turns out to hold, and a column that yields nothing still draws nothing. Worlds whose terrain never stacks generate identically.

Added — Java API

• SurfaceAnchor.resolveYBelow(PlacementContext, BlockPos, int) — resolve an anchor against the part of a column at or below a ceiling, which is how the walk reaches the second body and the ones under it. It is a default method with single-shot semantics (the anchor's own Y when that lies at or below the ceiling, otherwise nothing), so a variant that does not override it keeps reporting one body per column exactly as before. world_surface and world_floor override it; see docs/COMPATIBILITY.md.