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/loader.go

@@ -31,6 +31,13 @@ type OverworldDensity struct {
 	Temperature, Humidity, Continentalness, Erosion, Weirdness, Depth DensityFunction
 }
 
+// SurfaceRule returns the overworld surface rule tree, loading it on first use.
+// It does not depend on the seed. A nil rule (on error) is non-fatal: the
+// generator falls back to its default surface heuristics.
+func (od *OverworldDensity) SurfaceRule() (SurfaceRule, error) {
+	return LoadOverworldSurfaceRule()
+}
+
 // LoadOverworldFinalDensity builds the overworld final_density function for the
 // given world seed.
 func LoadOverworldFinalDensity(seed int64) (*OverworldDensity, error) {