Forward (push): When KOReader pushes a CREngine XPointer, convert it
to standard epubcfi before storing. Uses CFIConverter.ConvertCREToStandard
with context_text for text search fallback.
Reverse (pull): When KOReader pulls progress, convert stored standard
epubcfi back to CREngine XPointer via CFIConverter.ConvertStandardToCRE.
Returns as koreader_xpointer field in GetMetadata response.
Other changes:
- updateProgressForBook: pass context_text to SaveProgress
- enqueueProgressForBook: pass context_text to queue
- KOReaderProgressData: add KoreaderXPointer field
- New convertCFIToXPointer helper method
- Wire libraryService in main.go for EPUB path resolution