The full-wrap cover

One page carrying back cover, spine and front cover, with bleed on all four outside edges. That single sheet is what KDP wants uploaded.

python3 ${CLAUDE_PLUGIN_ROOT}/scripts/build-cover.py
python3 ${CLAUDE_PLUGIN_ROOT}/scripts/build-cover.py --pages 420   # what-if only

The rule the whole design hangs off

The cover is built last, and from the interior — never from a number.

Spine width is page count times caliper. A cover built before the interior was finished, or carried over from a previous edition, has a spine that does not match the book. At best the spine type sits off-centre; at worst the title wraps onto the front face.