Supersedes the plain case-insensitive matcher from earlier today (user
decision): an all-lowercase pattern searches case-insensitively, one
capital makes it exact (vim smartcase), and diacritics always fold both
ways — /ete finds été, /été finds ete. fold() strips the Latin-1
accent set (ligatures œ/æ excluded: they fold to two chars); n/N
recompute the smartcase verdict from the remembered pattern so repeats
behave like the original search.
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