Add idle-time glyph prewarming for next page

Preload glyphs for the next page during idle time to reduce page-turn latency. After a 400ms debounce period, scan the next page in FCM scan mode (no pixels drawn) to cache missing glyphs from SD card, avoiding ~100ms of SD reads during the actual page turn. Prewarm is deferred when rendering is active, heap is low, or during rapid page-flipping.

src/activities/reader/EpubReaderActivity.cpp

@@ -356,6 +356,35 @@ void EpubReaderActivity::loop() {
     }
   }
 
+  // Idle glyph prewarm for the likely next page (currentPage + 1). The scan
+  // pass draws nothing (FCM scan mode suppresses pixels), so the displayed
+  // framebuffer is untouched; endScanAndPrewarm loads only glyphs not already
+  // cached. Debounced past rapid page-flipping, one attempt per position, and
+  // deferred while a render/build owns the CPU or the heap is at the render
+  // floor. Cross-chapter prewarm is deliberately out of scope (next spine's
+  // section isn't loaded).
+  constexpr unsigned long IDLE_PREWARM_DEBOUNCE_MS = 400;
+  if (section && !section->isBuilding() && !RenderLock::peek() && renderer.hasFrameBuffer() &&
+      lastRenderCompleteMs != 0 && millis() - lastRenderCompleteMs > IDLE_PREWARM_DEBOUNCE_MS &&
+      ESP.getFreeHeap() > RENDER_MIN_FREE_HEAP &&
+      (idlePrewarmSpine != currentSpineIndex || idlePrewarmPage != section->currentPage)) {
+    idlePrewarmSpine = currentSpineIndex;
+    idlePrewarmPage = section->currentPage;
+    const int nextPage = section->currentPage + 1;
+    if (nextPage < static_cast<int>(section->pageCount)) {
+      RenderLock lock;  // the page table must not change under the scan
+      if (const auto p = section->loadPage(nextPage)) {
+        if (auto* fcm = renderer.getFontCacheManager()) {
+          const auto t0 = millis();
+          auto scope = fcm->createPrewarmScope();
+          p->render(renderer, SETTINGS.getReaderFontId(), 0, 0);  // scan only, no pixels
+          scope.endScanAndPrewarm();
+          LOG_DBG("ERS", "Idle prewarm: page %d in %lums", nextPage, millis() - t0);
+        }
+      }
+    }
+  }
+
   // Lazily resume a partial's extension build once the reader nears its watermark. Far from
   // it the rebuild is all cost (whole-chapter re-layout from page 0) and no benefit this
   // session, so reopening a partial deliberately does NOT start it (see the deferral in
@@ -1468,6 +1497,7 @@ void EpubReaderActivity::render(RenderLock&& lock) {
     const auto start = millis();
     renderContents(std::move(p), orientedMarginTop, orientedMarginRight, orientedMarginBottom, orientedMarginLeft);
     LOG_DBG("ERS", "Rendered page in %dms", millis() - start);
+    lastRenderCompleteMs = millis();
     // Fragmentation tracker: free vs largest block after every page. A falling
     // maxAlloc/free ratio across pages points at whichever allocation pattern the
     // preceding lines show (mini rebuilds, kern reloads, BLE churn).

src/activities/reader/EpubReaderActivity.h

@@ -46,6 +46,13 @@ class EpubReaderActivity final : public Activity {
   // to the chapter/book title the moment the connection completes (and
   // returns on disconnect) instead of waiting for the next page turn.
   bool statusBarBleConnected = false;
+  // Idle-time glyph prewarm: after a page settles, scan the LIKELY next page
+  // (scan mode draws nothing) and load its missing glyphs from SD during idle,
+  // so the next turn's in-render prewarm is a cache hit instead of ~100 ms of
+  // SD reads on the page-turn critical path. One attempt per position.
+  int idlePrewarmSpine = -1;
+  int idlePrewarmPage = -1;
+  unsigned long lastRenderCompleteMs = 0;
   bool bookmarkRemoved = false;  // true when last toggle removed (controls popup text)
   std::vector<BookmarkEntry> cachedBookmarks;
   // Tracks whether this book is currently removed from Recent Books by the