When TOC entries use fragment-suffixed hrefs (e.g. ch01.xhtml#ch01),
the sectionsMap lookup matched subitems with cfi: undefined instead of
parent sections with valid CFIs. This caused findTocItemBS to map every
annotation to the last TOC entry. Now skip subitems lacking a CFI and
fall back to the base section.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>