The arcade engine: /arcade is a game that looks like a page

/arcade looks and works exactly as it did. Underneath, it is a game.

• The world. A three.js scene on one transparent canvas over the page, measured in the page's own pixels, so anything put in it can sit on a tile and scroll with it. It takes no clicks.
• The page as pieces. The title, summary, shelf, each cover (cover:<slug>), the change machine and the gang carry data-arcade ids. A piece can be found, followed, moved and put back exactly as it was.
• Keys and pointer, for anything that wants steering later.
• Acts. src/app/arcade/engine/acts.ts is the list of things the world does, and it is empty. Each act can start at once, after a delay, on a roll of the dice, or after so many presses on a piece.

While the list is empty, nothing changes for a visitor: no canvas, no three.js download, no loop. Tested with a throwaway act (a cube riding the Sloth Splash cover through a scroll, the change machine tipped and put back, clicks still landing on the covers), then removed.

Next, not this ticket: the whole page becomes a neon arcade, with an exit at the top centre like leaving a theatre. This is what that gets built on.