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>

src/activities/ActivityResult.h

@@ -39,7 +39,9 @@ struct PageResult {
 
 struct SyncResult {
   int spineIndex = 0;
-  int page = 0;
+  int page = 0;                    // estimated page (fallback)
+  uint16_t paragraphIndex = 0;     // 1-based <p> index from XPath
+  bool hasParagraphIndex = false;  // true when paragraphIndex is available
 };
 
 enum class NetworkMode;