A pure module picks the pill's other side: production for a preview, the newest open preview for production #6075

Part of closed pull #5862, split out.

Done when:

• [ ] src/apps/side.ts exports sideOf(app, environment, previews) → {label, url} or null. A preview gets Open on production with production's address. Production gets Newest preview: #N <title> for the highest pull number among previews that are running or asleep, or null when there are none. Every address comes from the forge's record of the app, never from the request's Host.
• [ ] test/apps-side.test.ts covers both sides, no previews, and a stopped preview being skipped.

The branch of pull #5862 holds a version of this to start from.