`descendants(of:)` recursed on wrapped XPath nodes and, at every node, built
a fresh array for that node's subtree and concatenated it
(`result.append(contentsOf: descendants(of: child))`). That is an array
allocation and copy per node, roughly O(n x depth) work over the subtree, and
the parse profile showed it dominating XPath evaluation for a `//`-style query
over a wide document.
Recurse on the raw tree node instead, appending into one shared inout
accumulator. The result is the identical node-set in the identical document
order; only the way the array is built changes, from O(n x depth) to O(n).
Measured xpath ~46% faster on the 20k-item generated corpus via a same-state
A/B (count unchanged at 10000). Full suite, the Apache Xalan XSLT gold-output
corpus, and the WASM build stay green.
Mihaela Mihaljevic
cf8246a1cb