docs: introduce anime cyber kitsune mascot

[arthurpanhku]

Summary

• completely replace the former animal-with-shield mascot with an original adult anime cyber kitsune sentinel
• use fox ears, holographic data tails, high-collar cyber techwear, and a hand-controlled binary energy ring
• preserve the neon arcade and binary-data atmosphere without copying an existing game character
• keep the shared asset at 768x768 and update the primary README alt text and caption

Validation

• inspected the full-resolution PNG
• verified the actual 220px README thumbnail remains recognizable
• confirmed all localized READMEs continue to use the shared asset path
• ran git diff --check

Generation

Created from scratch with the built-in ImageGen workflow. No existing character image was used as an input. The prompt requires an original face, hairstyle, costume, pose, tail arrangement, and effects, and explicitly excludes recognizable characters, costumes, logos, or game UI.