Beyond The End - Optimization and Fixes

Speed up vertical feature scans without changing their results

- count_on_every_layer: scan each column once per getPositions call and record
  every ground instead of rescanning from the top for each layer. Nothing is
  placed while vanilla builds the position list, so the column cannot change.
  Random draws and their order are unchanged.
- environment_scan (vertical only): when both predicates only test block states
  at fixed offsets (state tests, true, not, all_of, any_of) and every tested
  position lies in an air-only section, both predicates are constant up to the
  section edge, so skip those steps at once. Every other step is vanilla.
- Both skip air-only sections, whose reads return AIR in ProtoChunk, LevelChunk
  and ImposterProtoChunk.
- Expose predicate structure through small duck-interface mixins.
- Verify mode compares both against the vanilla scan.