EPUB как второй вход в конвейер; листинги кода не переводятся и не ломают приёмку

- epub2html.py: EPUB -> тот же XHTML, что pdf2html.py, только стандартная
  библиотека. Уровень глав определяется по книге, а не берётся из <h1>:
  в EPUB там обычно название и части.
- pdf2html.py: листинг узнаётся по флагу моноширинного шрифта PyMuPDF,
  переносы и отступы внутри <pre> сохраняются, де-дефисация к коду не
  применяется.
- translate.py: <pre> исключён из проверок языка (английский код утягивал
  долю кириллицы ниже порога приёмки), рабочий каталог привязан к книге,
  устаревшие главы прошлого прогона чистятся, инлайновый <code> переживает
  переводчика.
- bookhtml.py: общий каркас документа и CSS для обоих входов.
- Тесты: test_epub2html.py на собранном в памяти EPUB, четыре новых случая
  в test_translate.py.

SKILL.md

@@ -15,6 +15,30 @@ tag, and 2817 body paragraphs became `<h2>`.
 properties, and emits semantic XHTML. calibre is then used only as the
 XHTML→EPUB packer.
 
+## Two entry points
+
+`scripts/pdf2html.py` for PDF, `scripts/epub2html.py` for EPUB. Both emit the
+same normalized XHTML — one block per line, `<pre>` for code listings — and
+everything downstream (translation, packing, audiobook) is identical. Shared
+document skeleton and CSS live in `scripts/bookhtml.py`.
+
+**Do not route an EPUB through the PDF path.** Converting EPUB→PDF→XHTML throws
+away the semantic markup that is already there and re-derives it from font
+sizes. `epub2html.py` needs no font profiling and no threshold tuning: it reads
+the spine, drops the nav document, and maps the book's own headings.
+
+Steps 1, 3 and 4 below are PDF-only. For EPUB start at step 2 (only calibre is
+needed, not PyMuPDF) and convert with:
+
+`python scripts/epub2html.py book.epub out/book.html`
+
+It reports which heading level turned out to be the chapter level. In most
+EPUBs `<h1>` is the book title and the parts, while chapters are `<h2>` — the
+script picks the deepest level that still gives a sane chapter count, because
+the translation bridge splits the book on `<h1>`. Measured on Stroustrup's PPP
+3rd edition: 7 `<h1>` against 49 `<h2>`, chapters correctly detected as `h2`,
+56 chapters out of 656 pages.
+
 ## Workflow
 
 1. **Verify a text layer exists.** `pdfinfo` and `pdffonts` on the file. No
@@ -35,7 +59,9 @@ XHTML→EPUB packer.
    Read off: the body font (largest character count), the italic variant, the
    heading sizes, and any secondary family used for sidebars, journal entries,
    chat logs or slides.
-4. **Tune the thresholds** in `block_kind()` and `style()` to that output. The
+4. **Tune the thresholds** in `block_kind()` and `style()` to that output. Code
+   listings need no tuning — a monospaced span is recognized by the PyMuPDF font
+   flag (`flags & 8`), not by font name, and becomes a `<pre>` block. The
    defaults target a 15pt/letter calibre layout: `>= 28` or a display font is a
    title, `>= 24` a chapter/part `h1`, `>= 17` an `h2` subtitle, a secondary
    family is `p.note`. `INDENT_X` (default 88) is the x-coordinate that
@@ -45,7 +71,10 @@ XHTML→EPUB packer.
 
    `python scripts/pdf2html.py book.pdf out/book.html`
 
-6. **Verify before packing** (see Verification). Fix thresholds and re-run until
+6. **Verify before packing** (see Verification). Check the `pre:` counter against
+   the real number of listings in the book — a technical book reporting `pre: 0`
+   means the mono flag never fired and every listing is about to be reflowed as
+   prose. Fix thresholds and re-run until
    the counts are sane. Cheap to iterate; do not skip to packing.
 7. **Pack with calibre**, always passing explicit TOC XPaths — without them
    calibre applies its own heuristics and re-breaks the chapters:
@@ -97,6 +126,23 @@ navPoint count for the TOC size.
 Only when the user asks for a translated book. It slots between step 6 and
 step 7 — translate the XHTML, then pack the translated file with calibre.
 
+**Code listings are never translated.** The bridge only recognizes `<p>`, `<h1>`
+and `<h2>`; a `<pre>` block is not parsed, and anything unparsed is carried into
+the result untouched. Nothing extra is needed to protect code — but this also
+means a listing that was misclassified as a paragraph upstream *will* be
+translated, which is the real reason step 6 checks the `pre:` counter.
+
+For the same reason `<pre>` is cut out of both language checks. A book that is
+40% listings translates correctly and would otherwise fail acceptance, because
+the English code drags the Cyrillic share below the threshold.
+
+**One workdir per book.** Chapter files are named `chapter_NNN.json` for every
+book, and the external repo skips chapters it has marked done — a reused workdir
+silently stitches one book's translation onto another's text. The bridge writes
+`bridge_source.json` into the workdir on first run and refuses to start if the
+directory belongs to a different book. Re-running the same book is unaffected;
+that is the resume path.
+
 Translation is delegated to an external project,
 [`vetermanve/book_translator`](https://github.com/vetermanve/book_translator)
 (DeepSeek or a local Ollama model). **Never modify that repository** — it is