docs: refresh autocrop logo assets

[leblancfg]

Updates the autocrop mark across the facelifted docs and README.

Changes:

• Adds a reusable filled-silhouette-and-crop-marks logo SVG.
• Updates the favicon to match the filled mark.
• Uses the logo in the docs header.
• Adds Open Graph and Twitter preview metadata.
• Adds an editable SVG social preview that matches the pi-fusion proportions, with the right-side graphic area left empty.
• Uses the SVG preview in the README until a Mac-rendered PNG is available.

Verification:

• Validated SVG XML for the logo, favicon, and social preview.
• Inspected the social preview layout.
• Ran git diff --check.
• Ran just check.