Fix: duck visible through alpha-transparent card videos

[ob1-s]

Problem

On cards playing the Pollen clips (e.g. alpha-walking), the static DuckMark remained visible through the video: the clips under moves-portrait-alpha/* are alpha-transparent WebMs, so their transparent regions composited over the always-present duck fallback layer. Cards never showed this before because they previously played local opaque MP4 loops.

Fix

Complete the layering design with a crossfade:

• when the media layer reveals (onLoad / canplay, hydration-race-proof via the existing ref sync), the duck fallback fades out in the same 240ms;
• whenever no media is paintable (slow network, load failure), the duck stays fully visible — the empty-frame protection from Perf: layered duck fallback + official remote media for cards #5 is unchanged;
• reduced-motion users get instant swaps (no transitions).

The halftone dots of the card container remain the composite background for the transparent clips, consistent with the site motif.

Test plan

• pnpm test (17/17), pnpm build
• Static catalog HTML unchanged in structure: 0 <video>, 18 duck layers, 7 poster layers
• Manual: hard-refresh catalog — no duck-through-video on alpha clips; throttled connection still shows the duck while loading