Each open element resolved its name and attributes by looking up namespace
bindings through NamespaceContext.lookup, which walked a stack of scopes as
deep as the document. With one lookup per element over an O(depth) stack,
parsing a deeply-nested document was O(depth^2): a 40000-deep document took
~39s.
The context now maintains the current in-scope bindings incrementally: a
single prefix-to-URI map, plus a per-element undo list recording the
bindings each element overwrote so closing it restores the enclosing state.
A lookup is O(1) and entering/leaving an element is O(its xmlns
declarations), so parsing is O(n). The same document now parses in ~0.2s.
Namespace semantics are unchanged: the nearest in-scope binding still wins,
an empty default binding still undeclares, and 81 namespace tests plus the
full suite (1784) pass. Closes#345.
