fix(epub): prevent blank pages from corrupted index files

- Clean up incomplete temp files before retry attempts in Section.cpp
- Remove failed stream files immediately to prevent corruption
- Add data consistency validation in TextBlock deserialization (wc == xc == sc)
- Add sanity check for unreasonably large word counts (max 10000)
- Add iterator bounds validation before rendering to prevent overflow

This fixes an issue where pages after a certain point would appear blank
due to corrupted .bin files from failed SD card streaming retries.

lib/Epub/Epub/Section.cpp

@@ -127,12 +127,23 @@ bool Section::persistPageDataToSD(const int fontId, const float lineCompression,
       delay(50);  // Brief delay before retry
     }
 
+    // Remove any incomplete file from previous attempt before retrying
+    if (SD.exists(tmpHtmlPath.c_str())) {
+      SD.remove(tmpHtmlPath.c_str());
+    }
+
     File tmpHtml;
     if (!FsHelpers::openFileForWrite("SCT", tmpHtmlPath, tmpHtml)) {
       continue;
     }
     success = epub->readItemContentsToStream(localPath, tmpHtml, 1024);
     tmpHtml.close();
+
+    // If streaming failed, remove the incomplete file immediately
+    if (!success && SD.exists(tmpHtmlPath.c_str())) {
+      SD.remove(tmpHtmlPath.c_str());
+      Serial.printf("[%lu] [SCT] Removed incomplete temp file after failed attempt\n", millis());
+    }
   }
 
   if (!success) {

lib/Epub/Epub/blocks/TextBlock.cpp

@@ -4,11 +4,18 @@
 #include <Serialization.h>
 
 void TextBlock::render(const GfxRenderer& renderer, const int fontId, const int x, const int y) const {
+  // Validate iterator bounds before rendering
+  if (words.size() != wordXpos.size() || words.size() != wordStyles.size()) {
+    Serial.printf("[%lu] [TXB] Render skipped: size mismatch (words=%u, xpos=%u, styles=%u)\n", millis(),
+                  (uint32_t)words.size(), (uint32_t)wordXpos.size(), (uint32_t)wordStyles.size());
+    return;
+  }
+
   auto wordIt = words.begin();
   auto wordStylesIt = wordStyles.begin();
   auto wordXposIt = wordXpos.begin();
 
-  for (int i = 0; i < words.size(); i++) {
+  for (size_t i = 0; i < words.size(); i++) {
     renderer.drawText(fontId, *wordXposIt + x, y, wordIt->c_str(), true, *wordStylesIt);
 
     std::advance(wordIt, 1);
@@ -46,6 +53,13 @@ std::unique_ptr<TextBlock> TextBlock::deserialize(File& file) {
 
   // words
   serialization::readPod(file, wc);
+
+  // Sanity check: prevent allocation of unreasonably large lists (max 10000 words per block)
+  if (wc > 10000) {
+    Serial.printf("[%lu] [TXB] Deserialization failed: word count %u exceeds maximum\n", millis(), wc);
+    return nullptr;
+  }
+
   words.resize(wc);
   for (auto& w : words) serialization::readString(file, w);
 
@@ -59,6 +73,13 @@ std::unique_ptr<TextBlock> TextBlock::deserialize(File& file) {
   wordStyles.resize(sc);
   for (auto& s : wordStyles) serialization::readPod(file, s);
 
+  // Validate data consistency: all three lists must have the same size
+  if (wc != xc || wc != sc) {
+    Serial.printf("[%lu] [TXB] Deserialization failed: size mismatch (words=%u, xpos=%u, styles=%u)\n", millis(), wc, xc,
+                  sc);
+    return nullptr;
+  }
+
   // style
   serialization::readPod(file, style);