New <leaflet-control-layers> wraps Control.Layers. Children with
slot="base" or slot="overlay" and a name attribute are automatically
registered as base layers or overlays. The control intercepts child
leaflet-register events and stops propagation so layers are managed
by the layers control rather than added directly to the map.
Unchecked layers that connected before the control are removed from
the map before the control initialises, so Control.Layers._addItem
correctly reflects the unchecked state on first render.
New <leaflet-icon> element wraps L.Icon / L.DivIcon and dispatches
icon-changed custom events to parent markers. Marker listens for
the event and queries for existing icon children on connect.
Fix: use kebab-case attribute names (via PROPS table spec.attr) in
applyIcon() instead of camelCase, and guard against empty iconUrl
to avoid L.icon({}) throwing.
Buddy