Add click-to-play homepage launch video

[scheemunai]

Approved scope

• One current-system, reusable 4:3 / max780px marketing video after hero/setup and before Examples; registered in the style guide. Existing headline, CTAs, setup, fonts, icons, admin/detail/red charts, gallery and 20 canonical template/source/thumbnail options are unchanged.
• Exact optimized 46s/60fps deliveries: desktop4.12MB, phone2.84MB; responsive posters14.5/34.3KB. Four original delivery bytes plus one840-byte authored visual WebVTT alternative, not invented speech/sound captions. Full hashes/reproduction/provenance in docs/homepage-video.md.
• No media src before explicit Watch. Scroll/hover/focus are passive. Native controls, playsinline, unmuted-after-action, focus, retry, open-file and genuine no-JS link. One rendition selected/pinned by CSS frame width; no autoplay/loop/third-party service.
• Existing static adapter supplies real ranges. Narrow hashed-media immutable/MP4 range-advertisement and WebVTT MIME headers only; no Nginx, CSP, environment, launch-flag, database, dependency or provider change.

Verification

• pnpm check + typecheck/build pass:41 exact existing warning locations/rules, no added warnings/errors.
• Full unit/integration: 1,153 pass /12 existing skips. Full Chromium E2E: 338 pass /12 existing skips (28 new player cases across seven configured widths).
• Actual HTTP full hashes, HEAD, exact206 first1024bytes/suffix, MIME/cache, native5-cue parsing; positive/negative missing/416 tests.
• Fresh1440/768/390/320 and nine actual width-policy/gutter neighbors: zero passive movie requests, one poster/rendition, stable4:3 activation geometry. Eight matched hero/header/legal/catalog PNG pairs exact at1440/390.
• Five cold-phone full-length real-HTTP trials at1.6Mbps/150ms emulated latency: first-frame705.2/697.8/692.4/696.2/696.3ms (median696.3ms; worst705.2ms), no post-first-frame stalls. One run dropped8/2760frames; other four0. These precede only server-side VTT MIME correction; movie/player/CSS/markup identical; final HTTP/native-track tests verify that correction separately.

Limits / rollout

Chromium lab evidence, not Safari/iPhone/Firefox/deployed-CDN certification. No perceptual audio audition was available; objective audio alignment/decode checks are not listening. Small embedded text may need native fullscreen/transcript. Player activation CLS0 is separate from initial font swaps in the existing shell; no global-zero claim.

Review only: do not merge/deploy yet. Existing guarded deployment provides assets-before-page ordering. No homepage-status flip. Planned rollback is scoped code revert with immutable media retained for already-served references, then the same guarded deploy; no routine DB restore. No icon adoption or unrelated changes.