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 still claimed there was no placed/configured feature system and
that ores never generate below y=0; both stopped being true when the
stage-6 region replay landed. Document what the replay covers, which
decoration stays hand-written, the measured parity baseline, and the new
env-gated base-terrain diagnostic. README gains the same correction plus
the 966-cell base-terrain measurement.
