KDP print geometry

Every number here comes from scripts/kdp_geometry.py. Do not compute a spine width, a wrap dimension or a gutter minimum in your head or in a one-off snippet — call the module. A caliper written down in two places is a caliper that will eventually disagree with itself.

python3 ${CLAUDE_PLUGIN_ROOT}/scripts/kdp_geometry.py --list-trims
python3 ${CLAUDE_PLUGIN_ROOT}/scripts/kdp_geometry.py --pages 398 --trim 6x9
python3 ${CLAUDE_PLUGIN_ROOT}/scripts/kdp_geometry.py --pages 398 --json

Prefer a standard trim

KDP splits paperback trims into standard and non-standard. Standard sizes are available in every marketplace, are the only ones some distribution channels accept, and are what the print-cost tables are quoted against. Non-standard sizes print, but expanded distribution may refuse them and the economics are worse.