perf(reader): keep large documents responsive while scrolling

[Lanlan13-14]

Summary

• Opening or scrolling a Markdown file of a few hundred kilobytes stalls the reader, because the whole rendered document is laid out and drawn by a single text view.
• Documents past 24,000 characters are now split into paragraph-bounded chunks of at most 12,000 characters, and only the chunks near the viewport are mounted. Everything else stays a spacer, so the scroll range, table of contents and search still cover the whole document.
• Shorter documents, raw view and the editor preview are unchanged and keep the single text view.

Test plan

• ./gradlew test, lint, detekt and assembleRelease pass on the fork's CI.
• Open a Markdown file of a few hundred kilobytes and scroll quickly from top to bottom; the reader should keep up instead of freezing.
• Jump to a heading near the end from the table of contents and confirm it lands on that heading.
• Search for a word that occurs late in the document and step between matches.
• Switch between rendered and raw view and confirm the reading position is kept.
• Open a short Markdown file and confirm it renders exactly as before, including code blocks and tables.