Orb gallery P1 — publish from the editor (poster capture + submit)

[mabry1985]

Let a signed-in user publish the orb they're editing (plan: docs/internal/orb-gallery.md).

• Export → Publish action in the editor: capture the WebGL canvas to a poster PNG at a canonical pose; POST /api/orbs with the .orbis + poster.
• Server validates + stores (R2) + rows (D1); per-user rate limit.

Depends on P0 (#538).