Add the OpenCode Mark wallpaper

[vaprdev]

The OpenCode mark on a pixel checker floor, its fill rising and falling like liquid. Block edges land on cell boundaries, so they stay crisp in octant and quadrant terminals.

• Calm: one large mark in the lower right. Lively: lanes of smaller marks drift past, their fills sloshing. Teeming: marks build themselves from falling blocks and dissolve again, with a wave sweeping the checker.
• Clicks build a small mark and ripple the checker; a finished agent tops every fill up, an error drains them; the easter egg lights the checker up as one giant mark.

Verified: typecheck, dev/check.ts opencode1 in octant and quadrant modes and --verify-dirty (0 failed, up to 10 ms/frame, at most 12 cells of flicker), screenshots up to date.