fix(ads): banner truly continuous across navigation + re-measure on resize

The previous engine kept the scheduler running but the view re-`show()`-ed the
current message on every (re)mount, replaying the fade on each navigation — which
looked like the cycle restarting (especially for a single message). Now:

- A mounted AdBanner reads the engine's live message (bannerCurrent) and renders
  it immediately, with no fade; attach no longer re-shows. Only a real advance
  fades. Verified: opacity stays 1.0 across a navigation, message preserved.
- The fade is manual opacity on a .fadewrap layer (not transition:fade), kept
  independent of the scroll (inner track transform), so a long message still
  fades at both ends and a {#key} remount cannot force an intro fade.
- A viewport size change (portrait↔landscape) re-measures the current message
  (remeasureBanner on resize/orientationchange, debounced) so the scroll
  re-evaluates for the new width — the owner accepts the restart on resize.
  Rotator gains restart(); engine gains bannerCurrent()/remeasureBanner().

Engine continuity + remeasure unit-tested.

ui/src/lib/banner.ts

@@ -73,6 +73,9 @@ export interface BannerHost {
 export interface Rotator {
   start(): void;
   stop(): void;
+  /** Re-present the current message (re-measure overflow + restart its scroll), e.g. after the
+   *  viewport size changed. A no-op before start(). */
+  restart(): void;
 }
 
 /**
@@ -91,6 +94,7 @@ export function createBannerRotator(
   const sched = createScheduler(campaigns);
   let running = false;
   let cycleStart = 0;
+  let lastShown = ''; // the message currently presented, for restart() (re-measure)
   const timers: ReturnType<typeof setTimeout>[] = [];
 
   const at = (ms: number, fn: () => void) => {
@@ -104,6 +108,7 @@ export function createBannerRotator(
   function present(md: string) {
     if (!running) return;
     clear();
+    lastShown = md;
     host.show(md);
     at(config.fadeInMs, measure);
   }
@@ -151,6 +156,9 @@ export function createBannerRotator(
       running = false;
       clear();
     },
+    restart() {
+      if (running && lastShown) present(lastShown);
+    },
   };
 }