Complete a selection with the earlier chart-walk points it implies.

This exists so the CLIENT NEVER COMPARES SEQUENCE INDICES. The admin screen wants to tell an operator "3 selected, +5 earlier points included" before they commit, and the obvious way to get that number is a loop over sequence_index in JavaScript — which is precisely what this product does not allow anywhere, for a reason that outlives this screen: a client that knows how to order a walk is a client someone will later trust to enforce the order, and a test asserts the shipped client contains no such comparison.

So the arithmetic stays server-side and the screen asks for the answer. One cheap call on selection change, no ordering logic in the browser, and the same function (missing_trajectory_predecessors) that allocate refuses with — so the preview count and the commit can never disagree about what is required.