Demoscene: dissolve the cart swap instead of cutting

Paul: "could you fade between demoscene transitions it's very sudden". A second canvas holds a FROZEN copy of the outgoing cart's last frame, stacked over the live one and faded out by CSS over 1.4s while the incoming cart draws underneath.

• Proven: opacity 1 -> 0.89 -> 0.69 -> 0.48 -> 0.25 -> 0 across 1.4s, monotonic, with the frozen frame carrying real content throughout. demo-layer / bg-cart / explorer gates PASS.

▶ Play it: Stationary Weather — a long-form genre — put the phone in your pocket and it keeps playing https://stellate.app/?genre=ambient&seed=4567&path=2103.13280,2122.13313,2084.13313&xdur=13.122

1f8469e https://github.com/aboard-io/stellate/commit/1f8469eff29e0ceacbdc930d5a5d9c149731ac38