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/world/store.go

@@ -32,7 +32,7 @@ const dataVersion26 = 4790
 // first time it ran: chunkAt prefers the store over the generator, so the
 // already-explored area around spawn keeps its old terrain and every later fix
 // looks like it did nothing in exactly the place you are standing.
-const generatorVersion = 5
+const generatorVersion = 6
 
 // generatorVersionTag is the NBT key holding generatorVersion. It is namespaced
 // because it is ours, not part of the vanilla chunk format.