docs: attribute the residual parity gap to unreplayed pre-ore stages

The featureless capture settles what the 98.028% gap is made of: base
terrain is bit-exact, so the drift comes from air-writing features
(monster rooms, geode voids, lava lakes) that vanilla runs before the
ores and the region replay does not cover yet. Both files now point at
monster rooms as the next step instead of at carver and aquifer edges.

CLAUDE.md

@@ -163,10 +163,16 @@ Known gaps, roughly in order of how visible they are:
   outside the rule tree, for eroded badlands spires and frozen-ocean icebergs.
 
 Parity baseline (fixture seed 12345): biomes and heightmaps exact everywhere; blocks 95.378% through
-the single-chunk path, 98.028% through the production region replay. The remaining block gap is
-entirely underground and is driven by ~966 base-terrain cells (0.246%) where vanilla carved a cave
-segment we kept solid or an aquifer edge we sealed — those flips turn air-exposure discard rolls
-inside later veins, which is why ore drift looks symmetric (extra ≈ missing).
+the single-chunk path, 98.028% through the production region replay. A featureless vanilla capture
+(`cmd/vanillacapture -featureless -blocks-only`, biomes stripped to their carvers) proves the
+undecorated pipeline bit-exact against it — density, surface rules, carvers, aquifers, and
+noise-router veins match every one of the fixture's cells — so the residual block gap is entirely
+inside feature replay. Plain-state mismatches that once looked like base-terrain defects are
+air-writing features the replay does not cover yet: monster rooms (stage 3) open cave_air pockets,
+geodes leave voids, lava lakes fill stage-1 cavities. Vanilla runs those stages before the ores, so
+ore ellipsoids crossing that pre-existing air roll air-exposure discards our run never sees, which
+is why high-attempt ore drift looks symmetric (extra ≈ missing). The next step toward exact parity
+is replaying the remaining pre-ore stages, monster rooms first.
 
 ## Testing worldgen
 
@@ -198,8 +204,12 @@ The older optional
 Beyond the always-on tests, `internal/world` carries env-gated diagnostics for hunting the residual
 parity gap (all skip unless the variable is set): `REGIONIO_REGION_ORE_DIAGNOSTIC=1` compares the
 legacy / center-only / region-replay ore paths per state, and
-`REGIONIO_BASE_TERRAIN_DIAGNOSTIC=1` classifies fixture mismatches into base-vs-base defects (with
-sample coordinates), feature flips per family, and an ambiguous bucket for disk-shaped outputs.
-The base-vs-base count is the number to drive toward zero: each such cell can flip air-exposure
-discard rolls inside overlapping ore ellipsoids, so one sealed cave edge shows up as dozens of
-missing and extra ore blocks.
+`REGIONIO_BASE_TERRAIN_DIAGNOSTIC=1` classifies fixture mismatches by what could have written each
+side. `TestVanillaBaseTerrainParity` compares against `testdata/vanilla_base_12345.bin`, a capture
+of vanilla running with features and structure sets stripped out; regenerate it after any change to
+the derived-datapack logic:
+
+```
+go run ./cmd/vanillacapture -server server.jar -featureless -blocks-only \
+    -port 25585 -output internal/world/testdata/vanilla_base_12345.bin
+```