The bookmark filter, the Comments panel filter and the command palette
lowercased with str::to_lowercase, which follows no locale: "İSTANBUL"
became "i̇stanbul" (a combining dot added) and "ı" has no case pair, so
"istanbul" never found the bookmark "İSTANBUL ŞUBESİ" and "imzalayın" never
found a comment "Lütfen İMZALAYIN".
They now fold with pdfcraft_render::text::fold_case, the folding Find and
Search & Redact use (made public here): I, i, İ and ı count as one letter
and every other character folds to its own lowercase.