GHSA-965w-775f-mr7g: xmldom: Quadratic-memory consumption

## Summary When an element declares a namespace prefix, xmldom copies the entire in-scope namespace map into a fresh object and keeps that copy on the element while it is open on the parse stack. A crafted document that nests N elements, each declaring one unique prefix, therefore drives the parser to hold on the order of N(N+1)/2 = **O(N²) namespace-map entries at its peak**, so a small, highly compressible input exhausts the heap. …