feat(ads): carry the banner scroll position across navigation

Per the owner's idea: instead of moving the banner out of the per-screen header
(which would change its position), remember the banner's "life stage" and resume
it on the next screen. The engine already keeps the message + rotation timing;
this adds the scroll offset:

- bannerEngine tracks the in-flight scroll (target, duration, start). On attach,
  if a scroll is still running, it computes the current offset and calls the new
  host's resumeScroll(fromTx, toPx, remaining) — the view jumps to the carried
  offset and continues to the end over the remaining time, instead of restarting
  at the left.
- A finished scroll is left at its end; the rotator's own loop then takes over.

Verified: spot-checked in the browser (a long message at offset -785 resumes at
-788 on the next screen, not 0) and unit-tested (attach mid-scroll calls
resumeScroll with a partial offset and the remaining duration).

ui/src/lib/banner.ts

@@ -68,6 +68,10 @@ export interface BannerHost {
   overflowPx(): number;
   /** Animate the horizontal scroll to toPx over durationMs. */
   scrollTo(toPx: number, durationMs: number): void;
+  /** Resume an in-flight scroll on a freshly-mounted view: jump to fromTx (px, signed) instantly,
+   *  then continue to toPx over durationMs. Used by the engine on attach to carry the scroll
+   *  position across a navigation; the rotator itself never calls it. */
+  resumeScroll(fromTx: number, toPx: number, durationMs: number): void;
 }
 
 export interface Rotator {