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/world/biome_lookup.go

@@ -124,6 +124,14 @@ func BiomeAt(od *worldgen.OverworldDensity, wx, wz int) uint16 {
 	return biomeID(loadBiomeTable().FindBiome(point))
 }
 
+// BiomeNameAt returns the resolved surface biome NAME at block (wx, wz), for
+// surface-rule biome tests which match on name. It mirrors BiomeAt but skips
+// the name→ID→name round-trip the ID path would require.
+func BiomeNameAt(od *worldgen.OverworldDensity, wx, wz int) string {
+	point := worldgen.SampleColumn(od, SeaLevel, wx, wz)
+	return loadBiomeTable().FindBiome(point)
+}
+
 // BiomeAt3D returns the network biome ID for the biome cell containing block
 // (wx, wy, wz). s2D carries the five precomputed 2D climate axes for the column
 // (sampled once via SampleColumn2D); the 3D depth axis is evaluated at wy inside