Scan Age spawn columns from world top

src/main/java/net/dnorberg/rivencraft/book/AgeDimensionService.java

@@ -121,18 +121,12 @@ public final class AgeDimensionService {
 
         for (int x = -SURFACE_SEARCH_RADIUS; x <= SURFACE_SEARCH_RADIUS; x++) {
             for (int z = -SURFACE_SEARCH_RADIUS; z <= SURFACE_SEARCH_RADIUS; z++) {
-                int floorY = level.getHeight(Heightmap.Types.WORLD_SURFACE, x, z) - 1;
-                if (floorY < level.getMinBuildHeight() || floorY > maximumFloorY) {
+                Optional<BlockPos> columnFooting = highestFootingInColumn(level, x, z, maximumFloorY);
+                if (columnFooting.isEmpty()) {
                     continue;
                 }
 
-                BlockPos candidate = new BlockPos(x, floorY, z);
-                var state = level.getBlockState(candidate);
-                if (!level.getFluidState(candidate).isEmpty()
-                        || (!state.is(BlockTags.LEAVES)
-                        && !state.isFaceSturdy(level, candidate, net.minecraft.core.Direction.UP))) {
-                    continue;
-                }
+                BlockPos candidate = columnFooting.get();
 
                 if (best == null || candidate.getY() > best.getY()
                         || (candidate.getY() == best.getY()
@@ -145,6 +139,27 @@ public final class AgeDimensionService {
         return Optional.ofNullable(best);
     }
 
+    /** Scans downward from the top so cave floors cannot be mistaken for surface footing. */
+    private static Optional<BlockPos> highestFootingInColumn(ServerLevel level, int x, int z, int maximumFloorY) {
+        BlockPos.MutableBlockPos candidate = new BlockPos.MutableBlockPos(x, maximumFloorY, z);
+        for (int y = maximumFloorY; y >= level.getMinBuildHeight(); y--) {
+            candidate.setY(y);
+            if (!level.getFluidState(candidate).isEmpty()) {
+                return Optional.empty();
+            }
+
+            var state = level.getBlockState(candidate);
+            if (state.isAir()) {
+                continue;
+            }
+            if (state.is(BlockTags.LEAVES)
+                    || state.isFaceSturdy(level, candidate, net.minecraft.core.Direction.UP)) {
+                return Optional.of(candidate.immutable());
+            }
+        }
+        return Optional.empty();
+    }
+
     private static BlockPos fallbackSpawnFloor(ServerLevel level) {
         int floorY = Math.clamp(FALLBACK_PLATFORM_Y,
                 level.getMinBuildHeight(), level.getMaxBuildHeight() - 5);