Add paragraph index LUT for accurate KOReader position sync

Store per-page paragraph indices in section cache to enable precise
XPath-to-page and page-to-XPath mapping without reparsing XHTML.

Forward path (upload): generates XPath directly from paragraph LUT
instead of byte-offset estimation, eliminating drift in chapters
with non-uniform content density.

Reverse path (download): resolves incoming KOReader XPath p[N] to
the exact page via paragraph LUT lookup.

Paragraph counter counts all <p> elements including display:none
to match ChapterXPathIndexer and crengine's standard XPath counting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/KOReaderSync/ProgressMapper.h

@@ -8,9 +8,11 @@
  * CrossPoint position representation.
  */
 struct CrossPointPosition {
-  int spineIndex;  // Current spine item (chapter) index
-  int pageNumber;  // Current page within the spine item
-  int totalPages;  // Total pages in the current spine item
+  int spineIndex;                  // Current spine item (chapter) index
+  int pageNumber;                  // Current page within the spine item (estimated if no paragraph LUT)
+  int totalPages;                  // Total pages in the current spine item
+  uint16_t paragraphIndex = 0;     // 1-based <p> index from XPath (0 if unavailable)
+  bool hasParagraphIndex = false;  // True when paragraphIndex was resolved from XPath
 };
 
 /**