## Summary
* **What is the goal of this PR?** Add EPUB 3 support by implementing
native navigation document (nav.xhtml) parsing with NCX fallback,
addressing issue Fixes: #143.
* **What changes are included?**
- New `TocNavParser` for parsing EPUB 3 HTML5 navigation documents
(`<nav epub:type="toc">`)
- Detection of nav documents via `properties="nav"` attribute in OPF
manifest
- Fallback logic: try EPUB 3 nav first, fall back to NCX (EPUB 2) if
unavailable
- Graceful degradation: books without any TOC now load with a warning
instead of failing
## Additional Context
* The implementation follows the existing streaming XML parser pattern
using Expat to minimize RAM usage on the ESP32-C3
* EPUB 3 books that include both nav.xhtml and toc.ncx will prefer the
nav document (per EPUB 3 spec recommendation)
* No breaking changes - existing EPUB 2 books continue to work as before
* Tested on examples from
https://idpf.github.io/epub3-samples/30/samples.html
## Summary
* Consolidate chapter page data into single file
* Header structure of the file stays the same, following the page count,
we now put a LUT offset
* The page data is all then appended to this file
* Finally the LUT is appended to the end of the file, and the page count
is updated
* This will also significantly improve the duration of cache cleanup
which takes a while to scan the directory and cleanup content
* Remove page file version as it's all tied up into the section file now
* Bumped section file version to 7
* Moved section content into sub directory
* Updated docs
## Additional Context
* Benchmarks:
* Generating 74 pages of content from a chapter in Jade Legacy took:
* master: 6,229ms
* this PR: 1,305ms
* Speedup of 79%
* Generating 207 pages of content from Livesuit book:
* With progress bar UI updates:
* master: 24,250ms
* this PR: 8,063ms
* Speedup of 67%
* Without progress bar UI updates:
* master: 13,055ms
* this PR: 3,600ms
* Speedup of 72%
