Recover parts of the regression fiasco

Co-authored-by: Copilot <copilot@github.com>

lib/GfxRenderer/FontCacheManager.cpp

@@ -89,7 +89,6 @@ void FontCacheManager::recordText(const char* text, int fontId, EpdFontFamily::S
 
 FontCacheManager::PrewarmScope::PrewarmScope(FontCacheManager& manager) : manager_(&manager) {
   manager_->scanMode_ = ScanMode::Scanning;
-  manager_->clearCache();
   manager_->resetStats();
   manager_->scanText_.clear();
   manager_->scanText_.reserve(2048);  // Pre-allocate to avoid heap fragmentation from repeated concat
@@ -110,15 +109,13 @@ void FontCacheManager::PrewarmScope::endScanAndPrewarm() {
 
   manager_->prewarmCache(manager_->scanFontId_, manager_->scanText_.c_str(), styleMask);
 
-  // Free scan string memory
+  // Keep reserved capacity to avoid repeated alloc/free churn between pages.
   manager_->scanText_.clear();
-  manager_->scanText_.shrink_to_fit();
 }
 
 FontCacheManager::PrewarmScope::~PrewarmScope() {
   if (active_) {
     endScanAndPrewarm();  // no-op if already called (scanText_ is empty)
-    manager_->clearCache();
   }
 }