QUE-335: Rebuild the source-to-metric textbook around one worked document

The textbook now follows the KREF Q2 2026 supplement through every stage,
with page renders, stored records and live playground answers from that
document's real artifacts. Every claim was re-verified against main at
d54559d; corrections include the side-by-side declash, the switchable
ranker, nomination's own text-layer test, and value_unreconciled boxes.