Drop-cap markup like <p><span>C</span>onvergence of Heaven and Earth</p>
made getTextFromXPointer return just 'C'. The text search then matched
the first 'C' in the chapter and stored doc-start (/4/2/1:0) with
'precision: exact', so the web reader reopened at the chapter start
while the percentage looked mid-chapter.
- Add convertByStructuralPath: walk the parsed CRE ElementPath against
the raw XHTML (same-tag 1-based indexing, mirroring buildCREXPointer),
map CharOffset into the target element's text, and build the CFI.
Usable device text verifies the landing; disagreement falls through
instead of storing a confident-but-wrong CFI.
- Add usableContextText guard (>=8 runes, >=2 words): single chars can
never claim an exact text-search hit in either direction
(ConvertCREToStandard and reverseByTextSearch).
- Add drop-cap regression fixtures plus usable-context unit tests.
- Verified against the real book: DocFragment[26]/p[12]/span header now
converts to epubcfi(/6/52!/4/28/2/1:0) structural both with 'C' and the
full header, and round-trips back to DocFragment[26].
