Real badlands clay bands and a real biome temperature table

The bandlands rule cycled four terracotta colours off a per-column random draw.
Vanilla generates a 192-entry band table once per world, from a random source
named clay_bands, and reads it at the block's height shifted by the
clay_bands_offset noise. Brown, red and light grey terracotta were never placed
anywhere; the stripes were the wrong thickness and did not line up between
neighbouring columns. All seven colours now appear.

The temperature condition matched a hand-written list of eleven biome names.
Replacing it with the temperature field read out of the jar's 65 biome JSONs
fixes one of them: deep_frozen_ocean reads cold by name but its base
temperature is 0.5, so vanilla does not freeze it. taiga and the pine taigas
were the other way round -- excluded by name, and correctly so, but by
coincidence rather than by data.

Two parts of the vanilla calculation are left out and documented where they
belong: the height adjustment that cools peaks, and the "frozen" modifier that
warms scattered patches of frozen ocean. Both need PerlinSimplexNoise. Neither
is reachable from the overworld tree in a way that shows: the single condition
that consults temperature sits under a frozen_ocean biome check, below a water
check, and decides whether a hole in the ocean floor ices over. The snowy
mountain tops come from biome selection, not from here -- which is not what the
plan for this commit assumed.

The per-column *rand.Rand threaded through SurfaceContext goes away with the
old bandlands rule; nothing needs it now that vertical_gradient rolls
positionally.

internal/worldgen/loader.go

@@ -17,6 +17,7 @@ type Loader struct {
 	rs           *RandomState
 	dfCache      map[string]DensityFunction
 	interpolated []*Interpolated
+	bands        *clayBands
 }
 
 // OverworldDensity is the parsed final_density plus the set of Interpolated
@@ -446,3 +447,17 @@ func (l *Loader) parseSplineValue(v any) (DensityFunction, error) {
 	}
 	return l.parseNode(v)
 }
+
+// clayBands builds the world's badlands band table on first use. It is seeded
+// from the root positional factory hashed by name, as SurfaceSystem does.
+func (l *Loader) clayBands() (*clayBands, error) {
+	if l.bands != nil {
+		return l.bands, nil
+	}
+	offset, err := l.noiseField("minecraft:clay_bands_offset")
+	if err != nil {
+		return nil, fmt.Errorf("clay_bands_offset noise: %w", err)
+	}
+	l.bands = newClayBands(l.rs.Positional().FromHashOf("minecraft:clay_bands"), offset)
+	return l.bands, nil
+}