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/worldgen/surface.go

@@ -51,12 +51,18 @@ type SurfaceContext struct {
 	// Steep is true when the local slope exceeds the vanilla steep threshold
 	// (~1.0 surface-depth delta between neighbours).
 	Steep bool
-	// SurfaceDepth is the vanilla surface-depth value at this column (a small
-	// noise-driven integer 0..N) added to stone depth comparisons.
+	// SurfaceDepth is how thick the biome's surface layers are at this column
+	// (SurfaceSystem.getSurfaceDepth): usually 3, sometimes 0 or less, which is
+	// what "hole" tests for. It widens the stone_depth bands.
 	SurfaceDepth int
-	// PreliminarySurface is the top solid Y in this column; the
-	// above_preliminary_surface condition passes for blocks above it.
-	PreliminarySurface int
+	// SurfaceSecondary is the "minecraft:surface_secondary" noise at this
+	// column, which widens a stone_depth band further when the rule sets
+	// secondary_depth_range.
+	SurfaceSecondary float64
+	// MinSurfaceLevel is the lowest Y the biome surface subtree may reach:
+	// the interpolated preliminary surface level plus SurfaceDepth less 8.
+	// above_preliminary_surface tests Y against it.
+	MinSurfaceLevel int
 	// Rng is a per-column deterministic source for vertical_gradient and
 	// bandlands. It is seeded by the column so results are stable across runs.
 	Rng *rand.Rand
@@ -253,11 +259,11 @@ func (t stoneDepthTest) Test(ctx *SurfaceContext) bool {
 	if t.addSurfaceDepth {
 		surfaceDepth = ctx.SurfaceDepth
 	}
-	// Vanilla widens the band by map(surface_secondary noise, -1..1, 0..range).
-	// That noise is not sampled yet, so the secondary term stays 0; the two
-	// rules that use it also set add_surface_depth, and both currently reduce to
-	// the same single-block band either way.
-	return depth <= 1+t.offset+surfaceDepth
+	secondary := 0
+	if t.secondaryRange != 0 {
+		secondary = int(mapRange(ctx.SurfaceSecondary, -1.0, 1.0, 0.0, float64(t.secondaryRange)))
+	}
+	return depth <= 1+t.offset+surfaceDepth+secondary
 }
 
 // noiseThresholdTest passes when the named surface noise is within [min,max].
@@ -309,13 +315,12 @@ func (t verticalGradientTest) Test(ctx *SurfaceContext) bool {
 	return ctx.Rng.Float64() > float64(pos)/float64(band)
 }
 
-// abovePreliminarySurfaceTest passes for blocks at or above the column's
-// preliminary surface (the top solid Y). Vanilla gates the biome dispatch on
-// this so submerged blocks far below the surface keep stone.
+// abovePreliminarySurfaceTest gates the whole biome surface subtree: below the
+// column's minimum surface level nothing is dressed and the stone stays stone.
 type abovePreliminarySurfaceTest struct{}
 
 func (abovePreliminarySurfaceTest) Test(ctx *SurfaceContext) bool {
-	return ctx.Y >= ctx.PreliminarySurface
+	return ctx.Y >= ctx.MinSurfaceLevel
 }
 
 // ---- Parser ------------------------------------------------------------