docs(ui): correct the banner section to the shipped rotation behaviour

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.

docs/UI_DESIGN.md

@@ -215,14 +215,18 @@ a screen change does not replay the fade; only a real message advance fades. A *
 round-robin** (`createScheduler`) picks the next message: campaigns compete by their weight (each
 appears its weight share per cycle, evenly interleaved, not at random), and a campaign's own messages
 advance round-robin. One message fades in (`fadeInMs`), holds `holdMs` (a message wider than the
-strip pauses `edgePauseMs`, scrolls to its right edge at `scrollPxPerSec`, pauses, and repeats while
+strip pauses `edgePauseMs`, scrolls to its right edge at `scrollPxPerSec`, pauses, and — while still
-under `holdMs`), then — with more than one message — fades out (`fadeOutMs`), waits `gapMs`, and the
+under `holdMs` — fades out, rewinds to the start hidden, then fades the **same** message back in to
-next fades in. The fade (opacity on a `.fadewrap` layer) is independent of the scroll (the inner
+scroll again), then fades out (`fadeOutMs`), waits `gapMs`, and the next fades in. The fade (opacity
-track's transform), so a long, scrolling message still fades at both ends. A lone message stays put
+on a `.fadewrap` layer) is independent of the scroll (the inner track's transform). **Every** cycle
-(a long one keeps scrolling). A **viewport size change** (e.g. a portrait↔landscape rotation)
+boundary fades — a message change, a scroll rewind, and a **lone** message (which pulses out and back
-re-measures the current message (`remeasureBanner`, debounced), so its scroll re-evaluates for the
+in rather than sitting frozen) — so the rhythm is uniform. So the strip does not jump in height while
-new width. All timings are operator-set (`/_gm/banner-settings`). Under **reduce-motion** the fades
+the message is momentarily gone during `gapMs`, it reserves one text line with an invisible spacer
-collapse to an instant swap
+and overlays the message absolutely. When a navigation remounts the view mid-scroll, the engine
+**resumes the scroll from where it had reached** (`resumeScroll`) instead of restarting it at the
+left. A **viewport size change** (e.g. a portrait↔landscape rotation) re-measures the current message
+(`remeasureBanner`, debounced), so its scroll re-evaluates for the new width. All timings are
+operator-set (`/_gm/banner-settings`). Under **reduce-motion** the fades collapse to an instant swap
 and a long message does not scroll.
 
 ## Result / status iconography (`lib/result.ts`)