perf: Optimize HTML entities lookup to O(log(n)) (#1194)

## Summary

**What is the goal of this PR?** Replace the linear scan of
`lookupHtmlEntity` with a simple binary search to improve lookup
performance.

**What changes are included?**
`lib/Epub/Epub/Entities/htmlEntities.cpp`: 
 - Sorted the `ENTITY_LOOKUP` array.
 - Added a compile-time assertion to guarantee the array remains sorted.
 - Rewrote `lookupHtmlEntity` to use a binary search.

## Additional Context

Benchmarked on my x64 laptop (probably will be different on RISC-V)
```
=== Benchmark (53 entities x 10000 iterations) ===

Version           Total time   Avg per lookup
----------------------------------------------
linear          236.97 ms total     447.11 ns/lookup
binary search    22.09 ms total      41.68 ns/lookup

=== Summary ===

Binary search is 10.73x faster than linear scan.
```

This is a simplified alternative to #1180, focused on keeping the
implementation clean, and maintainable.

### AI Usage


Did you use AI tools to help write this code? _**< NO >**_

---------

Co-authored-by: Zach Nelson <zach@zdnelson.com>

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -761,7 +761,7 @@ void XMLCALL ChapterHtmlSlimParser::characterData(void* userData, const XML_Char
 void XMLCALL ChapterHtmlSlimParser::defaultHandlerExpand(void* userData, const XML_Char* s, const int len) {
   // Check if this looks like an entity reference (&...;)
   if (len >= 3 && s[0] == '&' && s[len - 1] == ';') {
-    const char* utf8Value = lookupHtmlEntity(s, len);
+    const char* utf8Value = lookupHtmlEntity(s, static_cast<size_t>(len));
     if (utf8Value != nullptr) {
       // Known entity: expand to its UTF-8 value
       characterData(userData, utf8Value, strlen(utf8Value));