feat(scripts): cached plugin bundle, single-plugin fetch, free-chapter prefix, nightly appender

- load-plugins: bundle once per source revision with an id index; loadPlugin(id)
  loads one plugin (144 ms / 87 MB instead of ~15 s / 1.2 GB).
- fetch-novel: keep only the contiguous run of free chapters (no locked teasers).
- update-novels: append newly free chapters to our EPUBs without moving positions.
- find-novel --json for programs; shared EPUB pieces in epub.js.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/epub.js

@@ -0,0 +1,28 @@
+// EPUB pieces shared by fetch-novel.js (new books) and update-novels.js (appends).
+import { load } from 'cheerio';
+import { stripPromo } from './clean-chapter.js';
+
+export const esc = s =>
+  String(s ?? '').replace(
+    /[&<>"]/g,
+    c => ({ '&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;' })[c],
+  );
+
+// Chapter HTML is tag soup; re-serialise it as well-formed XHTML without
+// scripts, remote resources (an EPUB must be self-contained) or promo lines.
+export const toXhtml = html => {
+  const $ = load(html, null, false);
+  $('script, style, iframe, link, meta, img, video, audio, source').remove();
+  stripPromo($);
+  return $.xml();
+};
+
+export const page = (title, body) => `<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html>
+<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="en" xml:lang="en">
+<head><meta charset="UTF-8"/><title>${esc(title)}</title></head>
+<body>${body}</body>
+</html>`;
+
+export const chapterPage = (title, body) =>
+  page(title, `<h2>${esc(title)}</h2>\n${body}`);