makeTextBook: UTF-8 decode (BOM stripped), blank-line paragraph split,
hard-wrapped lines joined (natural reflow under font-size changes), HTML
escaped, sections capped at ~150KB at paragraph boundaries (small Part N
TOC, avoids one giant DOM). Sections expose load/createDocument/size per
the reflowable interface; positions for txt books are percentage-based
(sections get fake CFIs from index, same as other non-EPUB books).
makeBook: dispatches text/plain or .txt in the final non-zip/non-pdf
branch. EPUB/PDF/CBZ/MOBI/FB2 routing untouched.
Tests: offline node harness (isText, paragraph join/split, escaping,
section capping, TOC shape, dispatch incl. zip-still-routes-to-EPUB).
