feat(text): plain-text (.txt) renderer

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).

view.js

@@ -117,6 +117,10 @@ export const makeBook = async (file, options = {}) => {
             const { makeFB2 } = await import('./fb2.js')
             book = await makeFB2(file)
         }
+        else {
+            const { isText, makeTextBook } = await import('./text.js')
+            if (isText(file)) book = await makeTextBook(file)
+        }
     }
     if (!book) throw new UnsupportedTypeError('File type not supported')
     return book