Jay Moran's display model is on main: src/display config, items, known and selections, with the arcade shelf they read #9274

Replaces #9207, now that its prerequisites are their own issues.

Done looks like:

• [ ] git checkout origin/incoming/jay-main -- src/display/config.ts src/display/items.ts src/display/known.ts src/display/selections.ts src/arcade/shelf.ts, adapting only imports to main.
• [ ] Nothing imports src/display/launch.ts, src/arcade/host.ts or src/arcade/snapshot.ts yet.
• [ ] test/display-model.test.ts reads a good display.json into items and selections, and refuses a malformed one with a named error.
• [ ] bun x tsc --noEmit and bun test pass. Commits carry Co-authored-by: Jay Moran.

Files: the five above and the test. Risks: do not touch src/http/api.ts. If tsc names another missing import, add exactly that export from his branch and say so in the pull.

duplicate of #9207