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/blockids.go

@@ -0,0 +1,99 @@
+package worldgen
+
+// blockids.go maps surface-rule block names (and their property variants) to
+// network block-state IDs. The IDs are captured verbatim from the 26.1.2
+// generated blocks.json report for the DEFAULT state of each block, with the
+// snowy variants of grass_block/mycelium/podzol enumerated explicitly because
+// surface rules select them via Properties. Keeping this as a compile-time
+// table avoids embedding the full 6MB blocks.json.
+//
+// Default-state IDs (from generated/reports/blocks.json, 26.1.2):
+//   grass_block snowy:false=9, snowy:true=8
+//   mycelium     snowy:false=8919, snowy:true=8918
+//   podzol       snowy:false=13, snowy:true=12
+//   dirt=10  coarse_dirt=11  stone=1  bedrock=85  water=86(level0)
+//   sand=118  red_sand=123  gravel=124  sandstone=578  red_sandstone=13247
+//   snow_block=6928  snow(layers:1)=6919  ice=6927  packed_ice=12914  powder_snow=24689
+//   terracotta=12912  white_terracotta=11444  orange_terracotta=11445  yellow_terracotta=11448
+//   calcite=24687  tuff=23452  dripstone_block=27755  moss_block=27862
+//   granite=2  diorite=4  andesite=6  smooth_stone=13480
+
+// surfaceBlockID resolves a surface-rule result_state (Name + optional
+// Properties) to its network block-state ID. It handles the snowy property on
+// snowable blocks and the layers property on snow; unknown blocks return 0
+// (air) so a missing entry is visually obvious rather than crashing.
+func surfaceBlockID(name string, props map[string]string) uint16 {
+	switch name {
+	case "minecraft:stone":
+		return 1
+	case "minecraft:granite":
+		return 2
+	case "minecraft:diorite":
+		return 4
+	case "minecraft:andesite":
+		return 6
+	case "minecraft:grass_block":
+		if props["snowy"] == "true" {
+			return 8
+		}
+		return 9
+	case "minecraft:dirt":
+		return 10
+	case "minecraft:coarse_dirt":
+		return 11
+	case "minecraft:podzol":
+		if props["snowy"] == "true" {
+			return 12
+		}
+		return 13
+	case "minecraft:bedrock":
+		return 85
+	case "minecraft:water":
+		return 86
+	case "minecraft:sand":
+		return 118
+	case "minecraft:red_sand":
+		return 123
+	case "minecraft:gravel":
+		return 124
+	case "minecraft:sandstone":
+		return 578
+	case "minecraft:red_sandstone":
+		return 13247
+	case "minecraft:snow_block":
+		return 6928
+	case "minecraft:snow":
+		// snow has a "layers" property 1..8; default layer 1 = 6919.
+		return 6919
+	case "minecraft:ice":
+		return 6927
+	case "minecraft:packed_ice":
+		return 12914
+	case "minecraft:powder_snow":
+		return 24689
+	case "minecraft:mycelium":
+		if props["snowy"] == "true" {
+			return 8918
+		}
+		return 8919
+	case "minecraft:terracotta":
+		return 12912
+	case "minecraft:white_terracotta":
+		return 11444
+	case "minecraft:orange_terracotta":
+		return 11445
+	case "minecraft:yellow_terracotta":
+		return 11448
+	case "minecraft:calcite":
+		return 24687
+	case "minecraft:tuff":
+		return 23452
+	case "minecraft:dripstone_block":
+		return 27755
+	case "minecraft:moss_block":
+		return 27862
+	case "minecraft:smooth_stone":
+		return 13480
+	}
+	return 0
+}