perf: reserve CSS rule map before loading from cache (#2435)

Co-authored-by: Erica Jensen <erica@mailershaven.com>

lib/Epub/Epub/css/CssParser.cpp

@@ -803,6 +803,9 @@ bool CssParser::loadFromCache() {
     return false;
   }
 
+  // Size the bucket array up front to avoid incremental rehashes while loading rules.
+  rulesBySelector_.reserve(ruleCount);
+
   auto hasRemainingBytes = [&file](const size_t neededBytes) -> bool {
     return static_cast<size_t>(file.available()) >= neededBytes;
   };