Mapping an ordered node-set to selections rebuilt each result as an
independent value Node through TreeNode.node, projecting that node's whole
subtree. For a result set whose nodes nest within one another (`//a` over a
deep chain, where every `a` contains all deeper ones) this materialized the
sum of their subtrees: quadratic. `//a` over an 8000-deep chain took ~19s.
The projection now threads a memo keyed by tree-node identity across one
result set. A tree node is projected once; a result that nests a
deeper one reuses its already-built value Node by reference, which the
copy-on-write Element storage (from the value-Node depth-safety work) makes
a shared, not copied, subtree. The set is built in one pass over the
distinct nodes. The same query now returns in ~0.04s and scales linearly.
The non-memoized `node` projection (serialization, value comparison) is
unchanged. Results are identical: 1787 tests pass, including the XPath and
XSLT value corpora. Closes#348.
Mihaela Mihaljevic
b672c49a84