perf: apply (micro) optimization on SerializedHyphenationPatterns (#689)

## Summary

This PR applies a micro optimization on `SerializedHyphenationPatterns`,
which allow reading `rootOffset` directly without having to parse then
cache it.

It should not affect storage space since no new bytes are added.

This also gets rid of the linear cache search whenever
`liangBreakIndexes` is called. In theory, the performance should be
improved a bit, although it may be too small to be noticeable in
practice.

## Testing

master branch:

```
english: 99.1023%
french: 100%
german: 97.7289%
russian: 97.2167%
spanish: 99.0236%
```

This PR:

```
english: 99.1023%
french: 100%
german: 97.7289%
russian: 97.2167%
spanish: 99.0236%
```

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? PARTIALLY - mostly IDE
tab-autocompletions

lib/Epub/Epub/hyphenation/SerializedHyphenationTrie.h

@@ -5,6 +5,7 @@
 
 // Lightweight descriptor that points at a serialized Liang hyphenation trie stored in flash.
 struct SerializedHyphenationPatterns {
+  size_t rootOffset;
   const std::uint8_t* data;
   size_t size;
 };