fix: remove OOM-causing hrefToSpineIndex map from TOC pass

The unordered_map with 2768 string keys (~100KB+) was causing OOM crashes
at beginTocPass() on ESP32-C3's limited ~380KB RAM.

Reverted createTocEntry() to use original O(n) spine file scan instead.
Kept the safe spineToTocIndex vector in buildBookBin() (only ~5.5KB).

lib/Epub/Epub/BookMetadataCache.cpp

@@ -48,6 +48,7 @@ bool BookMetadataCache::beginTocPass() {
     spineFile.close();
     return false;
   }
+
   return true;
 }
 
@@ -124,6 +125,18 @@ bool BookMetadataCache::buildBookBin(const std::string& epubPath, const BookMeta
   // LUTs complete
   // Loop through spines from spine file matching up TOC indexes, calculating cumulative size and writing to book.bin
 
+  // Build spineIndex->tocIndex mapping in one pass (O(n) instead of O(n*m))
+  std::vector<int16_t> spineToTocIndex(spineCount, -1);
+  tocFile.seek(0);
+  for (int j = 0; j < tocCount; j++) {
+    auto tocEntry = readTocEntry(tocFile);
+    if (tocEntry.spineIndex >= 0 && tocEntry.spineIndex < spineCount) {
+      if (spineToTocIndex[tocEntry.spineIndex] == -1) {
+        spineToTocIndex[tocEntry.spineIndex] = static_cast<int16_t>(j);
+      }
+    }
+  }
+
   ZipFile zip(epubPath);
   // Pre-open zip file to speed up size calculations
   if (!zip.open()) {
@@ -150,14 +163,7 @@ bool BookMetadataCache::buildBookBin(const std::string& epubPath, const BookMeta
   for (int i = 0; i < spineCount; i++) {
     auto spineEntry = readSpineEntry(spineFile);
 
-    tocFile.seek(0);
-    for (int j = 0; j < tocCount; j++) {
-      auto tocEntry = readTocEntry(tocFile);
-      if (tocEntry.spineIndex == i) {
-        spineEntry.tocIndex = j;
-        break;
-      }
-    }
+    spineEntry.tocIndex = spineToTocIndex[i];
 
     // Not a huge deal if we don't fine a TOC entry for the spine entry, this is expected behaviour for EPUBs
     // Logging here is for debugging
@@ -248,19 +254,15 @@ void BookMetadataCache::createTocEntry(const std::string& title, const std::stri
     return;
   }
 
-  int spineIndex = -1;
-  // find spine index
-  // TODO: This lookup is slow as need to scan through all items each time. We can't hold it all in memory due to size.
-  //       But perhaps we can load just the hrefs in a vector/list to do an index lookup?
+  int16_t spineIndex = -1;
   spineFile.seek(0);
   for (int i = 0; i < spineCount; i++) {
     auto spineEntry = readSpineEntry(spineFile);
     if (spineEntry.href == href) {
-      spineIndex = i;
+      spineIndex = static_cast<int16_t>(i);
       break;
     }
   }
-
   if (spineIndex == -1) {
     Serial.printf("[%lu] [BMC] addTocEntry: Could not find spine item for TOC href %s\n", millis(), href.c_str());
   }