Measure the surface water condition against the column's own water

The water condition asked "is this block at or above sea level", which is only
the same question as vanilla's in an ocean. Vanilla asks how far the block sits
below the water directly above it, and there is now water that is nowhere near
y=63: the aquifer puts pools at their own levels, deep underground and up in the
hills. Against sea level every one of those read as dry stone, and the stone
above them read as lakebed.

waterHeight is already tracked down the column, so the condition becomes the
vanilla one: pass when there is no water above at all, otherwise when
blockY (+ stoneDepthAbove where the rule asks for it) clears
waterHeight + offset + surfaceDepth * multiplier. add_stone_depth was parsed and
then ignored; three rules in the overworld tree set it.

NoWaterAbove replaces a bare math.MinInt so a hand-built context cannot default
to "water at y=0" by leaving the field unset.

internal/worldgen/surface_test.go

@@ -34,9 +34,11 @@ func TestSurfaceRuleNoPanic(t *testing.T) {
 	for _, b := range biomes {
 		for y := 0; y < 100; y++ {
 			ctx := &SurfaceContext{
-				X: 100, Y: y, Z: 100, StoneDepthAbove: 100 - y,
+				X: 100, Y: y, Z: 100,
+				StoneDepthAbove: 100 - y, StoneDepthBelow: y + 1,
 				SeaLevel: 63, BiomeName: b, MinY: -64,
-				PreliminarySurface: 100, Rng: rand.New(rand.NewSource(1)),
+				PreliminarySurface: 100, WaterHeight: NoWaterAbove,
+				Rng: rand.New(rand.NewSource(1)),
 			}
 			rule.Apply(ctx) // must not panic
 		}
@@ -51,9 +53,10 @@ func TestSurfaceBedrockFloor(t *testing.T) {
 		t.Fatalf("load: %v", err)
 	}
 	ctx := &SurfaceContext{
-		X: 0, Y: -64, Z: 0, StoneDepthAbove: 0,
+		X: 0, Y: -64, Z: 0, StoneDepthAbove: 1, StoneDepthBelow: 1,
 		SeaLevel: 63, BiomeName: "minecraft:plains", MinY: -64,
-		PreliminarySurface: 70, Rng: rand.New(rand.NewSource(1)),
+		PreliminarySurface: 70, WaterHeight: NoWaterAbove,
+		Rng: rand.New(rand.NewSource(1)),
 	}
 	state, ok := rule.Apply(ctx)
 	if !ok {
@@ -105,3 +108,64 @@ func TestIsColdBiome(t *testing.T) {
 		}
 	}
 }
+
+// TestWaterCondition pins SurfaceRules.WaterConditionSource against the
+// column's own water surface. The offsets are the three forms the overworld
+// tree actually uses: (0,0,false) for "is this block dry", (-1,0,false) for the
+// block just under the waterline, and (-6,-1,true) for the beach/shore band.
+func TestWaterCondition(t *testing.T) {
+	cases := []struct {
+		name string
+		test waterTest
+		ctx  SurfaceContext
+		want bool
+	}{
+		{
+			name: "no water above passes",
+			test: waterTest{},
+			ctx:  SurfaceContext{Y: 20, WaterHeight: NoWaterAbove},
+			want: true,
+		},
+		{
+			name: "at the waterline passes",
+			test: waterTest{},
+			ctx:  SurfaceContext{Y: 63, WaterHeight: 63},
+			want: true,
+		},
+		{
+			name: "one block under water fails",
+			test: waterTest{},
+			ctx:  SurfaceContext{Y: 62, WaterHeight: 63},
+			want: false,
+		},
+		{
+			name: "offset -1 reaches one block deeper",
+			test: waterTest{offset: -1},
+			ctx:  SurfaceContext{Y: 62, WaterHeight: 63},
+			want: true,
+		},
+		{
+			name: "an aquifer pool at y=-20 is measured against itself, not sea level",
+			test: waterTest{},
+			ctx:  SurfaceContext{Y: -25, WaterHeight: -20, SeaLevel: 63},
+			want: false,
+		},
+		{
+			name: "stone above the same pool is dry",
+			test: waterTest{},
+			ctx:  SurfaceContext{Y: -19, WaterHeight: -20, SeaLevel: 63},
+			want: true,
+		},
+		{
+			name: "add_stone_depth counts buried stone towards the threshold",
+			test: waterTest{offset: -6, surfaceDepthMul: -1, addStoneDepth: true},
+			ctx:  SurfaceContext{Y: 55, WaterHeight: 63, StoneDepthAbove: 2, SurfaceDepth: 0},
+			want: true,
+		},
+	}
+	for _, c := range cases {
+		if got := c.test.Test(&c.ctx); got != c.want {
+			t.Errorf("%s: got %v, want %v", c.name, got, c.want)
+		}
+	}
+}