Pick a corner and fire some toasts. Each corner anchor carries its own
data-⚓️-side (top corners stack below, bottom corners stack above)
and cross-axis data-⚓️-align-*. Toasts chain: the newest anchors to the corner, every
older one anchors to the toast in front of it. Switch corners and the whole stack
tweens across the screen (data-⚓️-tween + morph). JS only re-links anchor names.