feat(docx): Word (.docx) support via mammoth (lazy npm dependency)

makeDocxBook: the docx blob goes straight to mammoth.convertToHtml
(mammoth unzips internally; the browser entry takes {arrayBuffer}, the
node entry {buffer}). Converted HTML is sectioned at block-element
boundaries into ~150KB chunks (string-level — no DOMParser), giving a
Part N TOC; images inline as data URIs by default.

makeBook: zip files whose entries contain word/document.xml route to
the docx branch before the EPUB fallback (previously they died inside
new EPUB(...) on the missing container.xml).

mammoth is the fork's first runtime dependency: updates and security
fixes flow via semver; the browser-field mappings swap its node unzip
for the browser one; the dynamic import keeps it out of every other
format's path.

text.js

@@ -36,10 +36,6 @@ export const makeTextBook = async file => {
     if (cur.length) chunks.push(cur)
 
     const urls = new Map()
-    const makeHTML = paras => `<!DOCTYPE html><html><head><meta charset="utf-8">`
-        + `</head><body>`
-        + paras.map(p => `<p>${escapeHTML(p)}</p>`).join('\n')
-        + `</body></html>`
     const load = i => {
         if (!urls.has(i)) {
             const html = chunks[i].map(p => `<p>${escapeHTML(p)}</p>`).join('\n')