serialize_node/serialize_children, collect_text_inner (textContent), and
import_node_from recursed once per DOM nesting level with no depth cap, so a
deeply nested tree overflowed the thread stack and aborted the process. A
stack overflow is a hard abort that op_dom's catch_unwind cannot recover, so
one hostile page could take down the worker. descendants() was already made
iterative and capped for exactly this reason; these paths were missed.
Rewrite all three as iterative walks over an explicit heap stack and add the
same defense-in-depth cap so a cyclic graph stops instead of spinning.
Serialized output and traversal order are unchanged. Adds tests that build a
100k-deep tree and exercise each path.
Closes#364.
