fix: address review findings on prewarm gating and gitignore

Revalidate the section under RenderLock before the idle prewarm scan,
add a largest-block floor to its heap gate, rebuild the differential
baseline when the grayscale scratch allocation fails after an async
refresh, and anchor generated-file gitignore patterns to the repo root.

.gitignore

@@ -27,7 +27,7 @@ lib/EpdFont/scripts/output/
 !.claude/skills/
 /managed_components
 /.dummy
-CMakeLists.txt
+/CMakeLists.txt
-dependencies.lock
+/dependencies.lock
-sdkconfig.default
+/sdkconfig.default
-sdkconfig.defaults
+/sdkconfig.defaults

src/activities/reader/EpubReaderActivity.cpp

@@ -285,13 +285,17 @@ void EpubReaderActivity::loop() {
   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 &&
+      ESP.getFreeHeap() > RENDER_MIN_FREE_HEAP && ESP.getMaxAllocHeap() > BACKGROUND_BUILD_MIN_MAX_ALLOC &&
+      (idlePrewarmSpine != currentSpineIndex || idlePrewarmPage != section->currentPage)) {
+    RenderLock lock;  // the page table must not change under the scan
+    // Re-check under the lock: peek() and acquisition are not atomic, so the render
+    // task may have reset/replaced the section or moved the page in between.
+    if (section && !section->isBuilding() &&
         (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();
@@ -303,6 +307,7 @@ void EpubReaderActivity::loop() {
         }
       }
     }
+  }
 
   // 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
@@ -1526,6 +1531,13 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
       renderer.waitRefreshComplete();
       if (!scratch) {
         LOG_ERR("ERS", "OOM: grayscale strip scratch (%d bytes); skipping AA this page", gwBytes * STRIP_ROWS);
+        if (overlapRefresh) {
+          // The BW refresh ran the shadow-free async path, so controller RAM's
+          // differential baseline was never rebuilt. Even with AA skipped it must
+          // be re-synced from the intact BW framebuffer, or the next differential
+          // update diffs against stale contents.
+          renderer.cleanupGrayscaleWithFrameBuffer();
+        }
       } else {
         // Bands may be streamed in any order: X4 windows each via setRamArea,
         // X3 via PTL.