feat: add multi-spine chapter caching for seamless cross-spine navigation

When loading a section, proactively indexes all spine items belonging
to the same TOC chapter so page-turning across spine boundaries within
a chapter is instant. Uses Section::readCachedPageCount() to skip
already-cached sections and shows an "Indexing (x/y)" progress popup.

Ported from upstream PR #1172, adapted to mod architecture.

Made-with: Cursor

src/activities/reader/EpubReaderActivity.cpp

@@ -892,6 +892,58 @@ bool EpubReaderActivity::silentIndexNextChapterIfNeeded() {
   return true;
 }
 
+void EpubReaderActivity::cacheMultiSpineChapter(const uint16_t vpWidth, const uint16_t vpHeight) {
+  if (!epub || !section) return;
+
+  const int tocCount = epub->getTocItemsCount();
+  if (tocCount == 0) return;
+
+  const int currentTocIdx = section->getTocIndexForPage(section->currentPage);
+  if (currentTocIdx < 0) return;
+
+  const int startSpine = epub->getSpineIndexForTocIndex(currentTocIdx);
+  if (startSpine < 0) return;
+
+  // Chapter ends where the next TOC entry begins on a different spine
+  int endSpine = epub->getSpineItemsCount();
+  for (int t = currentTocIdx + 1; t < tocCount; t++) {
+    const int tSpine = epub->getSpineIndexForTocIndex(t);
+    if (tSpine > startSpine) {
+      endSpine = tSpine;
+      break;
+    }
+  }
+
+  // Collect uncached spines in this chapter
+  std::vector<int> uncached;
+  uncached.reserve(endSpine - startSpine);
+  for (int i = startSpine; i < endSpine; i++) {
+    if (i == currentSpineIndex) continue;
+    auto cached = Section::readCachedPageCount(epub->getCachePath(), i, SETTINGS.getReaderFontId(),
+                                               SETTINGS.getReaderLineCompression(), SETTINGS.extraParagraphSpacing,
+                                               SETTINGS.paragraphAlignment, vpWidth, vpHeight,
+                                               SETTINGS.hyphenationEnabled, SETTINGS.embeddedStyle,
+                                               SETTINGS.imageRendering);
+    if (!cached) uncached.push_back(i);
+  }
+
+  if (uncached.empty()) return;
+
+  const int total = static_cast<int>(uncached.size());
+  char buf[64];
+  for (int idx = 0; idx < total; idx++) {
+    snprintf(buf, sizeof(buf), "%s (%d/%d)", tr(STR_INDEXING), idx + 1, total);
+    GUI.drawPopup(renderer, buf);
+
+    Section s(epub, uncached[idx], renderer);
+    if (!s.createSectionFile(SETTINGS.getReaderFontId(), SETTINGS.getReaderLineCompression(),
+                             SETTINGS.extraParagraphSpacing, SETTINGS.paragraphAlignment, vpWidth, vpHeight,
+                             SETTINGS.hyphenationEnabled, SETTINGS.embeddedStyle, SETTINGS.imageRendering)) {
+      LOG_ERR("ERS", "Multi-spine cache failed for spine %d", uncached[idx]);
+    }
+  }
+}
+
 // TODO: Failure handling
 void EpubReaderActivity::render(RenderLock&& lock) {
   if (!epub) {
@@ -1011,6 +1063,10 @@ void EpubReaderActivity::render(RenderLock&& lock) {
       section->currentPage = newPage;
       pendingPercentJump = false;
     }
+
+    const uint16_t vpW = renderer.getScreenWidth() - orientedMarginLeft - orientedMarginRight;
+    const uint16_t vpH = renderer.getScreenHeight() - orientedMarginTop - orientedMarginBottom;
+    cacheMultiSpineChapter(vpW, vpH);
   }
 
   renderer.clearScreen();

src/activities/reader/EpubReaderActivity.h

@@ -40,6 +40,10 @@ class EpubReaderActivity final : public Activity {
   bool silentIndexingActive = false;
   bool silentIndexNextChapterIfNeeded();
 
+  // Multi-spine chapter caching: proactively indexes all spine items that belong
+  // to the same TOC chapter so cross-spine page turns are instant.
+  void cacheMultiSpineChapter(uint16_t vpWidth, uint16_t vpHeight);
+
   // Footnote support
   std::vector<FootnoteEntry> currentPageFootnotes;
   struct SavedPosition {