Node-set ordering previously recomputed both nodes' root paths on every
precedes comparison (treePath is O(depth x siblings), so sorts were
O(n log n x depth)). Add decorate-sort-undecorate helpers,
sortedByDocumentOrder() and firstInDocumentOrder(), that compute each node's
order key once, and switch the evaluator's order-and-dedup, the EXSLT set
functions, string-value coercion, and NodeFunctions' target lookup to them.
set:leading/trailing also decorate once, reusing the keys for the pivot
filter and the final sort. precedes stays for pairwise use, sharing the
key comparator (Node.ordered). Behavior unchanged, all suites green.
Closes#113
Add the EXSLT common/math/sets modules, dispatched by resolving a function's
prefix to its namespace (FunctionTable.call falls back to namespace lookup),
so any prefix bound to an EXSLT namespace works. math:min/max/highest/lowest/
abs/sqrt (with the NaN rule), set:distinct/difference/intersection/
has-same-node/leading/trailing (by node identity and document order), and
exsl:object-type. math:power and the transcendentals are omitted (they need a
C math library the Foundation-free target excludes); exsl:node-set awaits
result-tree-fragment support.
Part of #105 (Tier 3)
