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.

src/SdCardFontSystem.h

@@ -32,6 +32,9 @@ class SdCardFontSystem {
   /// Non-const access to the registry (for FontInstaller).
   SdCardFontRegistry& registry() { return registry_; }
 
+  /// Resident heap held by loaded SD fonts (audit; see SdCardFont::reportMemory).
+  size_t reportFontMemory() const { return manager_.reportMemory(); }
+
   /// Mark the registry as needing re-discovery.
   /// Thread-safe: can be called from the web server task.
   void markRegistryDirty() { registryDirty_.store(true, std::memory_order_release); }

src/activities/reader/EpubReaderActivity.cpp

@@ -38,6 +38,7 @@
 #include "fontIds.h"
 #include "util/BookmarkUtil.h"
 #include "util/ScreenshotUtil.h"
+#include "SdCardFontSystem.h"
 
 namespace {
 // pagesPerRefresh now comes from SETTINGS.getRefreshFrequency()
@@ -1502,6 +1503,20 @@ void EpubReaderActivity::render(RenderLock&& lock) {
     // maxAlloc/free ratio across pages points at whichever allocation pattern the
     // preceding lines show (mini rebuilds, kern reloads, BLE churn).
     LOG_DBG("MEM", "post-render: free=%u maxAlloc=%u", (unsigned)ESP.getFreeHeap(), (unsigned)ESP.getMaxAllocHeap());
+    {
+      // Heap audit: attribute resident heap to its owners so margin work is
+      // measurement-driven. `other` = IDF/Arduino baseline + SdFat + epub
+      // metadata + BLE (when on) + anything not yet instrumented.
+      const size_t fontBytes = sdFontSystem.reportFontMemory();
+      const size_t sectionBytes = section ? section->residentBytes() : 0;
+      const size_t fbBytes = renderer.hasFrameBuffer() ? renderer.getBufferSize() : 0;
+      const uint32_t heapTotal = ESP.getHeapSize();
+      const uint32_t heapFree = ESP.getFreeHeap();
+      LOG_DBG("MEM", "audit: total=%u free=%u fb=%u fonts=%u section=%u building=%d ble=%d other=%d", heapTotal,
+              heapFree, (unsigned)fbBytes, (unsigned)fontBytes, (unsigned)sectionBytes,
+              section && section->isBuilding() ? 1 : 0, BleHid.isRunning() ? 1 : 0,
+              (int)(heapTotal - heapFree - fbBytes - fontBytes - sectionBytes));
+    }
   }
   // Only persist when the position actually changed. render() also runs on menu,
   // bookmark and screenshot re-renders, and writeAtomic is several FAT ops for 6 bytes.