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/bannerEngine.ts

@@ -54,12 +54,31 @@ export function configureBanner(campaigns: BannerCampaign[], timings: BannerTimi
 }
 
 /**
- * attachBannerHost connects a freshly-mounted view as the DOM host and resyncs it to the live
- * message, so the banner resumes mid-cycle instead of restarting.
+ * bannerCurrent returns the message the engine is currently displaying (empty before the first
+ * one). A freshly-mounted view reads it to render the live message immediately, without a fade —
+ * so navigating between screens does not visibly restart the cycle.
+ */
+export function bannerCurrent(): string {
+  return current;
+}
+
+/**
+ * attachBannerHost connects a freshly-mounted view as the DOM host. It does NOT re-show the
+ * current message (the view renders it itself from bannerCurrent on mount): re-showing here would
+ * replay the fade-in on every navigation, which looks like the cycle restarting. The engine's own
+ * timer keeps driving show/hide for real message changes through this host.
  */
 export function attachBannerHost(host: BannerHost): void {
   mounted = host;
-  if (current) host.show(current);
+}
+
+/**
+ * remeasureBanner re-presents the current message (re-measuring overflow and restarting its
+ * scroll), for when the viewport size changed (e.g. a portrait↔landscape rotation) and a message
+ * that fit may now overflow, or vice versa.
+ */
+export function remeasureBanner(): void {
+  rotator?.restart();
 }
 
 /**