Update the fidelity notes for the surface pass

The gap list described a surface pipeline that no longer exists: no subsurface
banding, surfaceDepth pinned at zero, six unsupported noises, no deepslate.
What is left is smaller and more specific — carvers, ore veins,
PerlinSimplexNoise, and the two SurfaceSystem extensions that run outside the
rule tree.

Also records the two things that are now compile-time errors rather than silent
wrong output: a surface rule naming a block the ID table does not have, and the
rule tree being used without a seed.

CLAUDE.md

@@ -101,35 +101,34 @@ with trilinear interpolation, the climate/biome finder, and the chunk wire encod
 Ported from the decompiled source and checked by behaviour rather than by bytes — faithful as far as
 we can tell, but no vanilla capture confirms them: the aquifer (`worldgen/aquifer.go`, the whole of
 `Aquifer.NoiseBasedAquifer` bar `shouldScheduleFluidUpdate`), the surface-rule interpreter
-(`worldgen/surface.go`), and the column pass in `world/vanilla.go` that mirrors `SurfaceSystem`.
+(`worldgen/surface.go`, every condition the overworld tree uses), the badlands clay bands
+(`worldgen/bandlands.go`), and the column pass in `world/vanilla.go` that mirrors `SurfaceSystem`.
 
 The whole `noise_router` is parsed. `preliminary_surface_level` is reachable through
 `od.PreliminarySurfaceLevelAt`, which quart-aligns and memoises across chunks the way `NoiseChunk`
 does; the `vein_*` keys are parsed but nothing reads them yet.
 
+The rule tree is **seed-bound**: `od.SurfaceRule()` returns a `*SurfaceRuleSet` compiled against the
+world's `RandomState`, because `noise_threshold` and `vertical_gradient` cannot work without it. Get
+a context from `NewContext`, call `BeginColumn` per column, then `Apply` per block.
+
+Two things a surface rule cannot do silently: name a block that is not in `worldgen/blockids.go`
+(that is a parse error now — it used to resolve to 0 and get dropped, which is how deepslate went
+missing from the whole world), and be added without the world's height bounds (anchors resolve at
+parse time).
+
 Known gaps, roughly in order of how visible they are:
 
-- **`above_preliminary_surface` is wrong**, so there is no subsurface banding: every land column is
-  one grass block directly on stone, no dirt, no sandstone under sand. Vanilla is
-  `blockY >= preliminarySurfaceLevel + surfaceDepth - 8` with the level bilinearly interpolated from
-  the four corners of the 16-block cell; we compare against the actual top block, which gates the
-  whole biome surface subtree to a single block per column. `gendump` prints this. The router value
-  it needs is already available.
-- **`surfaceDepth` is always 0.** Vanilla is `surfaceNoise*2.75 + 3 + rand*0.25`, so the dirt band is
-  three-ish blocks deep; ours is one. This is the other half of the missing banding, and it also
-  neuters every `add_surface_depth`/`surface_depth_multiplier` term in the rule tree.
-- **Several surface-rule conditions are stubs**: `hole` is hardcoded false (vanilla is
-  `surfaceDepth <= 0`), `steep` is never assigned, the `minecraft:surface` noise is a per-column
-  random draw rather than the real noise, `surface_secondary` is not sampled at all (so
-  `secondary_depth_range` is ignored), and 6 of the 7 `noise_threshold` noises are unsupported so
-  calcite, ice, packed ice, powder snow, swamp water and gravel patches never appear. `bandlands` is
-  a 4-colour cycle rather than the 192-band array.
-- **`vertical_gradient` ignores absolute anchors**, reading only `above_bottom`. The bedrock floor
-  works because its anchors are `above_bottom`; the deepslate rule's are `absolute` 0..8, so both
-  collapse to y=-64 and **no deepslate is ever placed** — `gendump`'s deep-layer line reads
-  `deepslate=0` everywhere.
 - **No carvers and no ore veins.** Caves come only from the density router; `configured_carver` is
   not extracted and the `OreVeinifier` over the parsed `vein_*` keys is not written.
+- **No `PerlinSimplexNoise`**, so two corners of `Biome.coldEnoughToSnow` are missing: the height
+  adjustment that cools a column above sea level + 17, and the `frozen` temperature modifier that
+  warms patches of frozen ocean. Base temperatures are real (`worldgen/biome_temperature.go`,
+  extracted from the jar's 65 biome JSONs). The overworld tree reaches `minecraft:temperature` from
+  exactly one rule — whether a hole in a frozen ocean floor ices over — so neither omission is
+  visible; snowy peaks come from biome selection, not from this condition.
+- **`erodedBadlandsExtension` and `frozenOceanExtension` are not ported.** `SurfaceSystem` runs both
+  outside the rule tree, for eroded badlands spires and frozen-ocean icebergs.
 - **Decoration is hand-written heuristics**, not the vanilla feature system: oak trees only and
   without a biome check (so oaks grow in deserts), ores that cannot generate below y≈0 because they
   only replace stone and never deepslate, and no grass, flowers, lakes or springs.
@@ -138,14 +137,16 @@ Known gaps, roughly in order of how visible they are:
 
 `make verify` is the gate, but most generator defects are invisible to it — they show up as terrain
 that looks wrong. `cmd/gendump` exists for that: biome distribution, top surface blocks, subsurface
-banding, deep-layer composition, the bedrock band, the underground fluid census, and an ASCII
+banding, deep-layer composition, the bedrock band, the underground fluid census, the badlands clay
-cross-section, with no client involved. Add an assertion to it whenever you fix a class of defect;
+bands, and an ASCII cross-section, with no client involved. Add an assertion to it whenever you fix
-the bedrock-band check is the model — it prints per-layer counts and fails loudly on any air or
+a class of defect; the bedrock-band check is the model — it prints per-layer counts and fails loudly
-water in the floor. The fluid census is the same shape: it prints water as a share of the open
+on any air or water in the floor. The fluid census is the same shape: it prints water as a share of
-volume under inland chunks (3.8% now, 100% before the aquifer) and fails if caves flood again.
+the open volume under inland chunks (3.8% now, 100% before the aquifer) and fails if caves flood
+again.
 
-Anything gendump can assert on, prefer to also assert in a test — `TestCavesAreDry` and
+Anything gendump can assert on, prefer to also assert in a test. `internal/world` carries four that
-`TestNoFluidUnderBedrock` in `internal/world` are gendump checks that run under `make verify`.
+started as gendump checks and run under `make verify`: `TestCavesAreDry`, `TestNoFluidUnderBedrock`,
+`TestGrassColumnsHaveDirt`, `TestDeepslateLayer`.
 
 `go test -race` needs cgo and a C toolchain; on a Windows box without gcc, `make test-race` cannot
 run at all.