Functions and Change is still in progress

Keep the first math path compact: read parent-curve moves first, then local slope, and then accumulation so change stays graph-first all the way through.

This completion page stays public, but the completion reflection only unlocks after every concept and authored checkpoint in this track is clear on this browser. The route still reads the same local concept records and solved challenge ids, and signed-in sync keeps reusing those records instead of inventing a separate track store.