feat: PDF to Markdown, HTML, and Word

[Chimi6]

0.24 step 2.

• src/io/pdf/markdown.rs: blocks to Markdown events. Body size is the most common size; headings by larger sizes (three levels) and short all-bold lines below them; bullet and numbered list items; paragraphs joined with hyphenated words rejoined; page-number blocks dropped. Converters pdf-to-markdown, pdf-to-html, pdf-to-docx.
• Plain text dehyphenates as pdftotext does; blocks also join at the page's most common line spacing, so fonts whose glyph units are not the usual em (Ghostscript's CID rewrites) group right.
• Tests: the story and a report read as their Markdown; HTML carries the structure; Word reads back to the same Markdown. pdftotext match on 14 fixtures.
• Binary 2,539,488 (budget 2,550,000); wasm 1,270,010 (budget 1,300,000). The release will raise the budgets for the new paths.