The banner UX evolved across the PR2 polish iterations past what UI_DESIGN
described. Bring the section in line with the shipped behaviour:
- a lone message pulses (fades out and back in) instead of sitting frozen;
- a long message's scroll rewind fades (out → rewind hidden → fade the same
message back in) instead of every-message-only fades;
- the scroll position carries across a navigation (resumeScroll), rather than
restarting at the left;
- the strip reserves a constant line height (invisible spacer + absolute
message layer) so it does not jump while the message is gone during the gap.
ARCHITECTURE §10 already deferred these client-fade details to UI_DESIGN, so
this is the only doc that drifted. Docs-only, no code change.
Ilia Denisov