Restore the subsurface layers: real above_preliminary_surface and surface depth

Every land column was one block of grass sitting straight on stone. No dirt
under grass, no sandstone under sand, nothing. Two stubs did it together:

above_preliminary_surface compared blockY against the column's actual top block,
so of every position in the column exactly one passed -- and the entire
biome-specific half of the surface rule tree hangs under that condition.
Vanilla compares against a minimum surface level: the preliminary surface level
sampled at the four corners of the 16-block cell, bilinearly interpolated, plus
the surface depth less 8. That is about twenty blocks of reach on ordinary
terrain, which is what the biome subtree is written against.

Surface depth was hardcoded to 0. Vanilla is surfaceNoise*2.75 + 3 with a
per-column jitter, so it comes out around three; it sets how thick the band is
and feeds every add_surface_depth term in the tree. Zero collapsed them all.

Also samples surface_secondary, so stone_depth's secondary_depth_range widens
its band instead of being parsed and dropped.

Grass columns now read grass, two to four dirt, stone -- the histogram over 256
columns is {2: 223, 3: 33}, against vanilla's 2..4. gendump prints it and fails
if the band collapses again; TestGrassColumnsHaveDirt guards it in the suite.

internal/world/banding_verify_test.go

@@ -0,0 +1,67 @@
+package world
+
+import "testing"
+
+// TestGrassColumnsHaveDirt guards the subsurface banding. Before
+// above_preliminary_surface and surfaceDepth were real, every land column read
+// grass-on-stone: the biome subtree was gated to a single block per column and
+// the surface depth that widens the band was hardcoded to zero.
+//
+// Vanilla puts two to four blocks of dirt under the grass cap. The check is
+// deliberately a majority rather than a universal: a column on a steep slope or
+// in a surface "hole" legitimately has none.
+func TestGrassColumnsHaveDirt(t *testing.T) {
+	gen := NewVanillaGenerator(12345)
+	withDirt, total := 0, 0
+	depths := map[int]int{}
+	for cx := int32(-42); cx < -36; cx++ {
+		for cz := int32(-40); cz < -34; cz++ {
+			ch := gen(cx, cz)
+			for lx := 0; lx < 16; lx += 4 {
+				for lz := 0; lz < 16; lz += 4 {
+					topY, ok := grassTop(ch, lx, lz)
+					if !ok {
+						continue
+					}
+					total++
+					depth := 0
+					for y := topY - 1; y >= topY-6; y-- {
+						if ch.GetBlock(lx, y, lz) != StateDirt {
+							break
+						}
+						depth++
+					}
+					depths[depth]++
+					if depth >= 2 {
+						withDirt++
+					}
+				}
+			}
+		}
+	}
+	if total < 50 {
+		t.Fatalf("only %d grass columns found; the scan area has no land", total)
+	}
+	t.Logf("grass columns=%d with a dirt band>=2: %d; depth histogram %v", total, withDirt, depths)
+	if withDirt*4 < total*3 {
+		t.Errorf("only %d of %d grass columns carry a dirt band of 2+; the surface subtree is gated too tightly", withDirt, total)
+	}
+	if depths[6] > total/10 {
+		t.Errorf("%d of %d grass columns have 6+ blocks of dirt; the surface band is running away", depths[6], total)
+	}
+}
+
+// grassTop returns the Y of the column's grass cap, skipping decoration.
+func grassTop(c *Chunk, lx, lz int) (int, bool) {
+	for wy := MinY + WorldHeight - 1; wy >= MinY; wy-- {
+		switch b := c.GetBlock(lx, wy, lz); b {
+		case StateAir, StateWater, StateLava, StateOakLog, StateOakLeaf:
+			continue
+		case StateGrass:
+			return wy, true
+		default:
+			return 0, false
+		}
+	}
+	return 0, false
+}