Fix rounded transparent corners in branding

[offyotto]

The previous export kept opaque black pixels in the outer corners, so GitHub rendered the README logo as a square. This replaces the 512px branding asset with the original transparent-corner export (normalized to 512×512), and updates the matching site/logo asset.

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[gitar-bot]

Code Review ✅ Approved

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Replaces branding assets with transparent-corner exports to fix rounded corner rendering in the README logo and site. No issues found.

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