finish() derived the exact omitted count from the pre-trim prefix/suffix
budget, but the UTF-8 boundary trims drop additional partial-codepoint bytes,
so an "Omitted N bytes" notice overstated what was kept (head maxBytes:2 over
'a€b' returned 'a' but claimed exact 3 when 4 bytes are absent). Derive the
count from the bytes actually returned (total − keptPrefix − keptSuffix) so
exact metadata matches the text and atLeast stays a valid lower bound.
Dudu-0223
f4acb1cf05