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.

README.md

@@ -115,17 +115,17 @@ a complete decorated 3x3 neighborhood, while persisted neighbors still take
 precedence. This closes the chunk-lifecycle integration boundary without
 exposing undecorated chunks.
 
-The remaining block-parity gap is entirely underground: no surface or waterline
-cells mismatch. Parity diagnostics confirm the stage-6 feature schedule, the
-feature-index seeds, and the climate biome ordering are aligned with vanilla,
-and rare ores such as gold and redstone already match exactly. High-attempt
-features instead drift because a few hundred residual base-terrain differences
-(aquifer fluid edges and carver air boundaries) flip air-exposure discard rolls
-inside later veins; the base-terrain diagnostic measures them at 966 fixture
-cells (0.246%), concentrated in whole cave segments rather than isolated
-positions. The next worldgen milestone is to close those base-terrain
-differences and raise the 98.028% block parity toward exact equality while
-keeping cold batch generation within an acceptable latency budget.
+The remaining block-parity gap is entirely inside feature replay: a vanilla
+capture with every feature and structure set stripped out matches our
+undecorated pipeline on all 393,216 cells, so density, surface rules, carvers,
+aquifers, and noise-router veins are already bit-exact. What drifts is the
+interaction between stages — vanilla opens air pockets (monster rooms, geode
+voids, lava lakes) before the ores run, and ore ellipsoids crossing that
+pre-existing air roll air-exposure discard rounds our replay never sees, which
+is why high-attempt ore drift looks symmetric. The next worldgen milestone is
+to replay the remaining pre-ore stages, monster rooms first, and raise the
+98.028% block parity toward exact equality while keeping cold batch generation
+within an acceptable latency budget.
 
 ## Project layout