[yaroslav]

Moves Markdown serialization to pulldown-cmark-to-cmark 23 and fixes the bugs its stricter round-trip checks exposed.

• Breaking: Markdown output (to_markdown, chunks_by_heading, chunks_by_size, truncate_markdown) uses pulldown-cmark-to-cmark 23; escaping and formatting differ from 0.2.
• Fix raw HTML leaking into Markdown output with raw_html: false when a line held more than one tag.
• Fix chunks_by_size and truncate_markdown (at: :block) joining blocks without a blank line, which merged adjacent paragraphs.
• Fix Markdown output crashing on an image nested in another image's alt text with images: { lazy: true }, allowed_hosts, or allowed_schemes.
• chunks_by_heading starts sections only at top-level headings; a heading inside a block quote or list item no longer splits it.
• Raise Inkmark::Error on Markdown serialization failures instead of crashing the process.

Verified with a differential run against 0.2.2: every Markdown-emitting API over the 652 CommonMark spec examples and the repo's Markdown documents, across 9 option sets (30,150 calls). No errors, and no raw HTML in output when raw_html: false (0.2.2: 260 cases).