Add background video reference pack

[ael-dev3]

What changed

• packages the supplied Warpkeep background-video MP4 and four PNG references as one byte-exact, checksum-addressed release ZIP
• adds a high-resolution visual overview and six-frame video contact sheet
• records source metadata, provenance, licensing boundaries, release checksums, and a reproducible pack builder
• keeps the 18 MB source archive out of normal Git history

Why

This preserves the castle, menu, Core spire, and crest treatments as a coherent future-reference set for menu-background and cinematic-video creation.

Validation

• 5/5 supplied sources compare byte-for-byte with the ZIP contents
• ZIP integrity and internal SHA-256 checks pass
• release verifier accepts the 10-entry archive
• 35/35 repository tests pass
• independent rebuild produced a byte-identical ZIP

The asset release is reference-only; current runtime integration and final production status are not asserted.