The leaf chain is only trustworthy if it stays intact through every structural change. Today you prove it - hammer the tree with inserts and deletes that force splits and merges, then scan and confirm the chain still visits every key exactly once.
After a sequence of inserts and deletes that cause splits and merges, confirm a full scan returns exactly the surviving keys in order.
This lesson has little or no new code, and that is the point: it is the moment the leaf-link maintenance from the split and merge lessons gets proven end to end. Individually each operation re-pointed the links correctly; here they run together under churn, and a single missed link would show up immediately as a scan that skips a key or loops.
Walking the whole tree after splits and merges and getting back exactly the keys that should survive, in order, is the strongest evidence the chain is sound. A scan that returns them all is only possible if every split handed its forward link to the new right leaf and every merge inherited its neighbor’s link - the invariants you built earlier, now holding across a realistic sequence. This confidence is what lets the on-disk and crash-safety chapters lean on the chain without re-checking it.
// no new production code is expected - this lesson is a test that// exercises split and merge together, then walks the cursor/scan and// asserts the exact surviving key set in order. if it fails, the bug// is a next-leaf link not re-pointed during a split (l16) or merge (l25).