feat(docs): landing hero video (twd.dev + twd-js page) - #298

What

Adds the new 30s product hero video to the docs site:

• / (landing): hero screenshot replaced with a muted autoplay loop (docs/public/videos/twd-hero.mp4, 2.0 MB web-optimized, 1920×1080)
• /twd-js: LandingHero gains an optional video-src prop; the twd-js page opts in. twd-relay / contract-testing unchanged — future videos are one prop away.

Accessibility

• prefers-reduced-motion: reduce → no video mounts at all; the original static screenshot renders (WCAG 2.3.3)
• Pause/play control on every video (WCAG 2.2.2), aria-pressed, focus-visible
• SSR/no-JS → static screenshot (video mounts client-side only; screenshot doubles as poster)

About the footage

Every second is a real capture: Playwright recorded the TWD sidebar executing the actual twd-vue-example suite — nothing staged. Capture pipeline lives in twd-vue-example/video/.

Verification

• npm run docs:build green
• Playwright against the built site: video plays on / and /twd-js; toggle pauses; reduced-motion contexts render 0 videos + static img; /twd-relay unaffected