fix(registry): remove invalid editorial video placeholder

[miguel-heygen]

What

• remove the source-less background video placeholder from caption-editorial-emphasis
• remove the same unused placeholder from its demo
• lint the actual shipped registry component in a regression test

Why

hyperframes add caption-editorial-emphasis installed a component containing a timed <video> with no src. CLI 0.7.52 correctly raised media_missing_src, so the component could not be reused directly and had to be removed from the final composition.

How

The placeholder had no source and no runtime references, so the component now remains a transparent, host-agnostic caption overlay. The test reads the real registry artifact and asserts it has no media_missing_src finding.

Test plan

• reproduced via hyperframes add caption-editorial-emphasis followed by lint of the installed artifact
• watched the new regression test fail with media_missing_src before the change
• bun run --filter @hyperframes/lint test -- --run src/rules/media.test.ts (22 passed)
• bunx oxlint packages/lint/src/rules/media.test.ts
• bun run --filter @hyperframes/lint typecheck
• pre-commit lint, format, fallow, and typecheck hooks passed