✨ feat(serialize): add to_markdown GFM tree exporter

[gaborbernat]

turbohtml could pull raw .text off a node but had no structured text export, so the common scrape → Markdown pipeline still needed html2text or markdownify as a second dependency. to_markdown() closes that gap: it walks the existing arena tree once in C and emits GitHub-Flavored Markdown covering headings, lists, links, images, emphasis, strikethrough, inline and fenced code, blockquotes, horizontal rules, line breaks, and pipe tables. 📝

The design comes out of a source-level survey across languages — the Python html2text, markdownify, and inscriptis, JohannesKaufmann's Go html-to-markdown, and Rust's htmd, plus the C/C++ layout browsers w3m, lynx, and GNU html2text. They converge on a recursive DOM visit rather than a streaming parse, with the block context threaded through the recursion instead of re-derived by walking parent pointers. Whitespace is the hard part, so a run is never emitted eagerly; the owed space is deferred, and the emphasis markers are deferred with it, which moves a leading or trailing inner space outside the marker rather than producing the invalid **bold ** form. Plain prose is bulk-copied after its first character, the borrow-or-copy fast path htmd uses. Three spots the reference converters get wrong are correct here: an inline code span grows its backtick fence past any backticks in the content, a | inside a table cell is escaped, and a nested ordered list keeps its own counter on the recursion stack rather than a single field that corrupts on nesting.

Python stays a thin method that takes the same per-tree critical section .text and .html use. ⚡ The walk holds no state outside its stack frame, so it needs no mutex on the free-threaded build where Go's converter reaches for one. A new markdown benchmark suite measures to_markdown at 40× to over 100× faster than markdownify and html2text, validated against a markdown-it-py round-trip that renders the output back to HTML and checks no visible text was lost. Output is opinionated GFM with no options; layout-aware to_text(layout=...) (the inscriptis/w3m role) is a natural follow-up that the same walk can grow into.

closes #172