EPUB export of AZW3/MOBI: resolve TOC fragments and write embedded font assets

[Imaclean74]

Closes #22.

Summary

Two narrowly-scoped fixes to EpubExporter that turn currently-broken
output into correct EPUBs for AZW3 / MOBI books with fine-grained NCX
indexes and for any book with embedded fonts.

TOC fragment resolution (AZW3 / MOBI)

AZW3 / MOBI importers leave TOC entries with bare chapter hrefs at open
time; the #fileposN / #aid-XXXX fragment is filled in by
Book::resolve_toc. The exporter currently generates the NCX before
calling that, so every TOC entry within a single source chapter
collapsed onto the same partNNNN.html href.

Both export_raw and export_normalized now call book.resolve_toc()
before generating the NCX. EPUB importers don't need this (their TOC is
already resolved by the importer), so the call is a no-op for that
backend.

Font writing in normalized export

export_normalized writes assets from NormalizedContent::assets,
which only contains resources referenced from the IR DOM. Embedded
fonts are typically referenced from CSS @font-face rules and never
make it into the normalized asset list. The exporter now snapshots
book.list_assets() before normalization and, after writing the
normalized assets, enumerates every fonts/* path that wasn't already
covered — adding both an OPF manifest entry and a ZIP entry for each.

export_raw is unaffected here because it already writes the full
book.list_assets() set verbatim.

Related work

#13 surfaced KFX fonts via bcRawFont entity discovery; this PR makes
them survive the EPUB export. For AZW3 / MOBI font extraction, a
separate PR (#21) plumbs the Kindle FONT container decoder through to
list_assets. The font-writing logic here is format-agnostic and
benefits any importer that surfaces fonts/* paths.

Changes

• src/export/epub.rs:
  • export_raw: call book.resolve_toc() before NCX generation.
  • export_normalized: call book.resolve_toc() before NCX
    generation; snapshot book.list_assets() and, after writing the
    normalized-content assets, emit OPF manifest items + ZIP entries
    for any fonts/* path not already in content.assets.

Tests

tests/epub_exports_fonts_and_toc.rs:

• epub_export_writes_font_assets_from_kfx — uses the existing
  tests/fixtures/fonts_only.kfx.gz fixture, exports to EPUB, asserts
  the three KFX font assets are written into OEBPS/fonts/ and
  referenced in the OPF manifest.
• epub_export_resolves_azw3_toc_fragments — uses the existing
  tests/fixtures/epictetus.azw3 fixture, exports to EPUB, asserts
  the generated toc.ncx carries resolved #aid-XXXX fragments
  rather than bare chapter hrefs.

Both tests fail when reverting this patch and pass with it applied —
verified locally by git stash-ing the exporter changes and re-running.

Verification

```
cargo fmt -- --check
cargo clippy --lib --tests
cargo test --lib # 548 passed
cargo test --test epub_exports_fonts_and_toc # 2 passed
```