extract_readable_text recursed once per DOM nesting level, so a deeply
nested page overflowed the call stack and aborted the process (exit 134)
on --dump text. This is the CLI counterpart of the serialize/textContent
walks made iterative in obscura-dom; that fix did not cover this function.
Rewrite it as an iterative DFS over an explicit work stack, with a
Newline work item for a block element's trailing newline so output is
byte-for-byte unchanged, plus the same defense-in-depth node cap the DOM
walkers use. Relates to #362.
SGavrl
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