Add heap memory reporting for fonts and sections

Implements reportMemory() methods across SdCardFont, SdCardFontManager, and Section classes to track resident heap usage. Adds heap audit logging in EpubReaderActivity to attribute memory to fonts, framebuffer, section data, and other components for measurement-driven optimization.

lib/EpdFont/SdCardFontManager.cpp

@@ -88,6 +88,14 @@ void SdCardFontManager::unloadAll(GfxRenderer& renderer) {
   loadedPointSize_ = 0;
 }
 
+size_t SdCardFontManager::reportMemory() const {
+  size_t total = 0;
+  for (const auto& lf : loaded_) {
+    if (lf.font) total += lf.font->reportMemory();
+  }
+  return total;
+}
+
 int SdCardFontManager::getFontId(const std::string& familyName) const {
   if (familyName != loadedFamilyName_ || loaded_.empty()) return 0;
   return loaded_.front().fontId;