Follow-ups from craft-libs#485: `CHECK_FROM` is reset by a drop guard; `remember_missing` drops
the oldest of 256; a word none of whose letters the dictionary's words use (a Cyrillic word in
a Dutch run) is left alone, as Word does.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FypgCX8LsYYE8fsj4byBm2