Add EPUB book navigation frontmatter to Markdown export

## Problem

Docling already converts EPUB content well, but flattened Markdown loses:

- OPF title, author, date, language, and source metadata
- a random-access chapter index
- working links between EPUB spine documents

That leaves agents scanning the whole book or unpacking the EPUB just to land on a chapter.

## Proposed behavior

Add an opt-in EPUB Markdown mode that:

- writes YAML frontmatter with book metadata
- lists top-level chapter headings with absolute UTF-8 byte and 1-based line offsets
- uses fixed-width, leading-padded offset fields so frontmatter size is invariant
- rewrites internal EPUB document links to Markdown heading anchors

Defaults stay off, so existing output does not change.

## Acceptance

- every advertised byte and line lands on its heading
- repeat conversion is byte-identical
- no dangling `*.htm*#...` links remain
- ordinary EPUB Markdown and serialized document output remain unchanged

No file or test paths are named. Start by tracing the existing EPUB-to-Markdown conversion and serialized-document path, then locate handling for OPF metadata, spine documents, and internal links. Done means opt-in output meets every listed acceptance condition while ordinary Markdown and serialized output remain unchanged.

Written by the indexing model from the issue text.