v0.15.3

Fix: web-client-normalize-children now maps empty content '() to '() instead of wrapping it into a phantom empty text child (()). The phantom child kept an emptied element's old-records list non-null, suppressing reconcile-children!'s empty->populated bulk clear, which let foreign children of a reused element survive a full replacement. Treat '() like #f: no children.

Tests: new web-client-normalize-children group (empty / #f / content list / single atom), proven red on the pre-fix code.