docs: add real application screenshots

[Codewriter90x]

Summary

• adds two real first-party Unity captures to the README
• shows the current main menu and Concert Stage gameplay
• records both publication derivatives in the asset provenance register

Content boundary

• synthetic Neon Circuit demo only
• no commercial audio, chart, artwork, or performer likeness
• no Unity binary

Validation

• asset provenance contracts
• public readiness contracts
• website localization contracts
• Wiki contracts
• git diff --check