Add Video scroll-into-view autoplay and video compression tooling

[AdamGohs]

Summary

• Video component: autoplay now starts when the video scrolls into view and pauses when it scrolls away (IntersectionObserver), with a prefers-reduced-motion guard so it never autoplays for reduced-motion users. Playback is driven from JS rather than the native autoplay attribute for cross-browser consistency and WCAG 2.2.2 compliance. Preserves main's useBaseUrl asset-path resolution.
• Compression tooling: new npm run compress-video script (H.264/CRF via the bundled ffmpeg-static, no system install required). It re-encodes videos in place and preserves the untouched original as a gitignored *-raw.* backup. Supports --audio and --crf flags plus batch/glob/folder inputs.
• Documentation guide: documented the new Video autoplay behavior, GIF-replacement pattern, and WCAG controls guidance (ch. 07); added a "compress before committing" workflow note (ch. 06); and added a compress-video.js manual-utility note (ch. 17).

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📋 Lane: Dev Doc

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@usace-rmc/docs-admin please review, merge, and approve the deploy.

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