Add KEPUBCFIConverter in internal/sync that converts between KEPUB CFIs
(which include extra koboSpan DOM steps) and standard EPUB CFIs at sync
time, so only standard epubcfi values are stored in the database.
The converter works by:
1. Resolving the source CFI in the source document (EPUB or KEPUB)
2. Extracting surrounding text at the resolved position
3. Searching for that same text in the target document
4. Building a new CFI pointing to the matched text in the target
This text-content bridging handles the DOM structural differences
between EPUB (text nodes at depth 2) and KEPUB (text nodes wrapped in
<span class="koboSpan"> at depth 3).
Falls back to percentage-based positioning when text search fails,
matching the pattern used by the existing CRE converter.
No changes to existing cfi_converter.go or KOReader conversion code.
Same-package access to unexported functions (resolveCFIToNode, buildCFI,
findTextInNode, etc.) via internal/sync package placement.
john-okeefe
b0dc0c591e