Issue #3618086: Leave a notice alone when the answer repeats itself, so a redraw stops reading as news

Reconciles a topic's notice lines against what is on screen instead of dropping them and building them again, so an answer that repeats itself changes nothing: no empty frame, no flicker, and no redraw for a booker to read as news.

The scenario marks the line the first answer produced and looks for its mark after an identical second answer; it was seen to fail against the rebuild.

AI-Generated: Yes (Claude Code wrote the code and the test on this merge request. I reviewed and ran them before pushing.)