Pages like Express.co.uk live blogs nest 200+ DOM levels deep, overflowing
the default 1 MB main-thread stack on Windows during recursive markdown
conversion.
Two-layer fix:
1. markdown.rs: add depth parameter to node_to_md/children_to_md/inline_text
with MAX_DOM_DEPTH=200 guard — falls back to plain text collection at limit
2. lib.rs: wrap extract_with_options in a worker thread with 8 MB stack so
html5ever parsing and extraction both have room on deeply nested pages
Tested with Express.co.uk live blog (previously crashed, now extracts 2000+
lines of clean markdown) and drudgereport.com (still works correctly).
