Adds registerChildren / unregisterChildren / getChildren to
register.ts using module-scoped WeakMaps, removing the duplicated
#children / #childHandler fields + lifecycle wiring from 15
components.
registerChildren(el, layer) creates the children Map + handler,
stores both in WeakMaps, wires the event listener, and calls
registerWithParent — all in one call. unregisterChildren(el)
tears it down. getChildren(el) gives access to the map for
components that need custom child iteration in disconnectedCallback
(geojson, layer-group, feature-group).
Net: -101 lines, 15 components lose 2-4 fields + 4-8 lifecycle
lines each. Components with extra lifecycle work (marker -> dragend,
polygon/polyline -> observer + syncCoords) keep their custom
additions inline, just the register boilerplate is replaced.
e2752aa7e0