Skip safety rails for widgets with a large number of children

Nobody has claimed this yet.

Safety rails are a debug feature that panic early if they detect that a widget was implement incorrectly. They only run with `debug_assertions`, and overall documentation is careful to never *guarantee* that they'll run. They're here as an additional help for users.

When we're running with debug assertions, we assume that user doesn't care too much about performance and a small overhead is fine. However, safety rails can add large (think `O(N²)`) overhead to passes when widgets have a very large number of children.

We should add an automatic escape hatch for cases where `children_ids` is large, so we can skip children in those cases. We can assume that if `MyCustomWidget` has a bug in child handling, the will probably show up with a small number of children too.

Start by locating the safety-rail checks that process children_ids under debug assertions. Examine how child handling is validated and identify where a large-child-count escape hatch would apply. Define and verify the skip threshold and confirm that ordinary small-child cases retain their safety checks.

Written by the indexing model from the issue text.