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.

docs/UI_DESIGN.md

@@ -209,16 +209,20 @@ the profile to show or hide it in place.
 
 The rotation runs in a **persistent module engine** (`lib/bannerEngine`): the scheduler and timer
 live outside the components, so navigating between screens (which remounts the view) **continues the
-cycle** rather than restarting it — each mounted `AdBanner` only attaches as the DOM host and resyncs
-to the live message. A **smooth weighted 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 under `holdMs`), then — with more than one message —
-fades out (`fadeOutMs`), waits `gapMs`, and the next fades in. The fade (a `{#if}` `transition:fade`
-layer) is independent of the scroll (the inner track's transform), so a long, scrolling message
-still fades at both ends. A lone message stays put (a long one keeps scrolling). All timings are
-operator-set (`/_gm/banner-settings`). Under **reduce-motion** the fades collapse to an instant swap
+cycle** rather than restarting it. A mounted `AdBanner` reads the engine's live message at init and
+renders it **immediately, without a fade** (`bannerCurrent`), and attaches only as the DOM host — so
+a screen change does not replay the fade; only a real message advance fades. A **smooth weighted
+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
+under `holdMs`), then — with more than one message — fades out (`fadeOutMs`), waits `gapMs`, and the
+next fades in. The fade (opacity on a `.fadewrap` layer) is independent of the scroll (the inner
+track's transform), so a long, scrolling message still fades at both ends. A lone message stays put
+(a long one keeps scrolling). 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`)