Building a TreeNode from a value Node, projecting it back via `node`, and
releasing it each recursed one native frame per tree level, so a deeply
nested document overflowed the call stack (signal 10 around 30-60k deep,
and far shallower on a constrained stack such as a Swift Task). None of
these is fixed by the serializer, which was already iterative.
- `TreeNode(_:)` and `node`/`projectedChildren` now drive explicit
post-order work stacks (entity-reference splicing, doctype/namespace
dropping, and the root special-cases are unchanged).
- `deinit` flattens the subtree into a heap worklist before releasing, so
each node frees with empty children and the teardown stays flat. Only a
node the teardown solely owns (isKnownUniquelyReferenced) is flattened,
so a child still referenced elsewhere keeps its subtree.
- The validator walk and the XPath descendant axis were made iterative for
the same reason.
A 60000-deep tree now releases in well under 0.1s; 1778 tests pass and no
accepted output changes. Part of #341; the value Node's own recursive
release and XPath result ordering/dedup remain depth-bounded and tracked
there.
