docs: add audited Jumpgate brand assets

[ruizkinio]

Summary

• add a reproducible 1280x640 Jumpgate social card and README hero
• preserve the exact audited Bridge mark, backdrop, font, and OFL notice
• document source commit, hashes, render dimensions, and screenshot-redaction boundary

Validation

• visual inspection of the exact 1280x640 PNG
• Markdownlint on README and asset documentation
• XML parsing of all SVG sources
• PNG dimensions, pixel format, and metadata inspection
• Gitleaks directory scan (0 findings)
• git diff --check