Add official branding and attribution guidance

[EricSun0218]

Summary

• add the official OGA monogram assets for light, dark, and monochrome use
• render the mark at the top of both README files with GitHub theme switching
• document recommended, optional game and mod attribution without adding license obligations
• add Apache-2.0 NOTICE handling to NuGet packages

Verification

• GitHub Markdown API rendered both README files with picture sources, heading, and badges intact
• source and repository SVG SHA-256 hashes match exactly
• Release build: 0 warnings, 0 errors
• Release tests: 848 passed
• packed NuGet smoke: LICENSE and NOTICE present