Biome-aware surface rules from the vanilla rule tree

Replaces the biome-blind fillVanillaColumn heuristics with a full
interpreter for the overworld surface_rule tree (already embedded in
overworld.json): block/sequence/condition/bandlands rules plus all 11
condition tests (biome, steep, hole, water, temperature, y_above,
stone_depth, noise_threshold, not, vertical_gradient,
above_preliminary_surface).

- worldgen/blockids.go: name(+Properties)→network-ID table for surface
  blocks (grass/sand/terracotta/mycelium/podzol/coarse_dirt/sandstone/
  calcite/snow/ice/...), with snowy property variants.
- worldgen/surface.go: rule-tree parser + interpreter + SurfaceContext;
  LoadOverworldSurfaceRule caches the seed-independent tree.
- loader.go: OverworldDensity.SurfaceRule() exposes the parsed tree.
- biome_lookup.go: BiomeNameAt returns the biome name for biome tests.
- vanilla.go: samples the 2D climate + biome before column fill, threads
  the rule tree and biome name into fillVanillaColumn, and applies it
  top-down with stone as the default for non-matching (deeper) blocks.
  The above_preliminary_surface gate uses an inclusive bound so the top
  solid block reaches the biome dispatch.
- Performance: one per-column RNG and a reused SurfaceContext keep the
  overhead to ~+13ms/chunk (71ms vs 58ms baseline), within the gate.

internal/worldgen/surface_test.go

@@ -0,0 +1,107 @@
+package worldgen
+
+import (
+	"math/rand"
+	"testing"
+)
+
+// TestLoadSurfaceRule confirms the embedded overworld surface_rule parses into
+// a rule tree without error. This guards the parser against any rule/condition
+// type the overworld uses.
+func TestLoadSurfaceRule(t *testing.T) {
+	rule, err := LoadOverworldSurfaceRule()
+	if err != nil {
+		t.Fatalf("LoadOverworldSurfaceRule: %v", err)
+	}
+	if rule == nil {
+		t.Fatal("nil surface rule")
+	}
+}
+
+// TestSurfaceRuleNoPanic runs the full rule tree across a range of Y values and
+// several biomes to confirm Apply never panics on real-world inputs. A panic
+// during generation would crash the server.
+func TestSurfaceRuleNoPanic(t *testing.T) {
+	rule, err := LoadOverworldSurfaceRule()
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	biomes := []string{
+		"minecraft:plains", "minecraft:desert", "minecraft:forest",
+		"minecraft:badlands", "minecraft:snowy_plains", "minecraft:ocean",
+		"minecraft:mushroom_fields", "minecraft:wooded_badlands",
+	}
+	for _, b := range biomes {
+		for y := 0; y < 100; y++ {
+			ctx := &SurfaceContext{
+				X: 100, Y: y, Z: 100, StoneDepthAbove: 100 - y,
+				SeaLevel: 63, BiomeName: b, MinY: -64,
+				PreliminarySurface: 100, Rng: rand.New(rand.NewSource(1)),
+			}
+			rule.Apply(ctx) // must not panic
+		}
+	}
+}
+
+// TestSurfaceBedrockFloor confirms the bottom of the world resolves to bedrock
+// (the vertical_gradient bedrock_floor rule is the first rule in the tree).
+func TestSurfaceBedrockFloor(t *testing.T) {
+	rule, err := LoadOverworldSurfaceRule()
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	ctx := &SurfaceContext{
+		X: 0, Y: -64, Z: 0, StoneDepthAbove: 0,
+		SeaLevel: 63, BiomeName: "minecraft:plains", MinY: -64,
+		PreliminarySurface: 70, Rng: rand.New(rand.NewSource(1)),
+	}
+	state, ok := rule.Apply(ctx)
+	if !ok {
+		t.Fatal("no rule matched at bedrock floor")
+	}
+	if state != 85 { // bedrock
+		t.Errorf("bedrock floor state = %d, want 85", state)
+	}
+}
+
+// TestSurfaceBlockIDResolution checks the block-ID table covers the blocks the
+// overworld surface_rule references, including snowy property variants.
+func TestSurfaceBlockIDResolution(t *testing.T) {
+	cases := []struct {
+		name  string
+		props map[string]string
+		want  uint16
+	}{
+		{"minecraft:bedrock", nil, 85},
+		{"minecraft:grass_block", nil, 9},
+		{"minecraft:grass_block", map[string]string{"snowy": "true"}, 8},
+		{"minecraft:mycelium", nil, 8919},
+		{"minecraft:podzol", map[string]string{"snowy": "true"}, 12},
+		{"minecraft:terracotta", nil, 12912},
+		{"minecraft:red_sand", nil, 123},
+		{"minecraft:coarse_dirt", nil, 11},
+		{"minecraft:calcite", nil, 24687},
+	}
+	for _, c := range cases {
+		if got := surfaceBlockID(c.name, c.props); got != c.want {
+			t.Errorf("surfaceBlockID(%q,%v) = %d, want %d", c.name, c.props, got, c.want)
+		}
+	}
+}
+
+// TestIsColdBiome confirms the snow-cover predicate recognises cold biomes so
+// the temperature condition routes snowy biomes to snow.
+func TestIsColdBiome(t *testing.T) {
+	cold := []string{"minecraft:snowy_plains", "minecraft:frozen_peaks", "minecraft:grove"}
+	for _, b := range cold {
+		if !isColdBiome(b) {
+			t.Errorf("isColdBiome(%q) = false, want true", b)
+		}
+	}
+	warm := []string{"minecraft:desert", "minecraft:plains", "minecraft:badlands"}
+	for _, b := range warm {
+		if isColdBiome(b) {
+			t.Errorf("isColdBiome(%q) = true, want false", b)
+		}
+	}
+}