Repair corrupted transparent logo assets

[offyotto]

PR #90 accidentally committed a malformed PNG blob, which is why GitHub renders the 512px icon with visual corruption. This replaces the three affected 512px branding assets with a verified 512×512 RGBA export that preserves the transparent rounded corners. README continues to reference icon-512.png, so it will render the rounded icon once this merges.

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[gitar-bot]

Code Review ✅ Approved

🟢 Low risk

Replaces three corrupted 512px branding assets with verified 512×512 RGBA exports that preserve transparent rounded corners, fixing the visual corruption in GitHub's icon rendering. No issues found.

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