Explore Taste Reveal

Open the reward the "import 3 reels" contract promised, and serve the board it produces in the same round trip. A reveal IS the Explore board, so it takes no surface.

409 below the target: the reveal is a promise kept, so a client that lost track of the count must not be able to open it early. Idempotent — the IS NULL guard keeps the first stamp, which is what cta.state is read from.