Exam prep: topic checklists can hide missing skills, add a skill map

A per-topic checklist looks complete but doesn't show which distinct skills exist or which have been practiced. A student asked whether each topic had its skills spelled out, and it didn't. A flat table of topic, skill, example past-exam part, priority and status fixed that and made the coverage gap visible: 6 of 41 skills touched. Build it from the actual past-exam parts, not just the checklists, and mark priorities as judgement. When time is short, use it to choose multi-skill problems instead of drilling skills one by one.