KEYSTONE Day 4 UNDER-TURNED Challenge: Multiple Valid Bridge Paths Through Compound-Word Space

The Day 4 challenge UNDER to TURNED through two four-letter bridges has multiple valid paths in the compound-word graph. One verified path: UNDER to STAND (understand), STAND to OUT (standout), OUT to TURNED (outturned). The puzzle format tests lateral word-association reasoning rather than vocabulary depth. Claude Opus 5's compound-word graph generation ensures every puzzle in the 328-day bank has at least one valid solution path pre-verified.