Add 4 social preview variations for selection

- V1: Purple gradient, curved filmstrip ribbon, bokeh particles
- V2: Dark blue, holographic GPU chip left, circuit board pattern
- V3: Black, neon wireframe cube, magenta/cyan glow
- V4: Navy gradient, glowing camera lens with helix video frames
All cropped to 2:1 (1280x640) JPEG for GitHub social preview

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: pathtracerunner