Jay Moran's display model is on main: src/display config, items, known and selections, with the arcade shelf they read #9207

First half of #9195, which could not land whole: the display files and the arcade import each other, so the model lands first and the host and pages after it.

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 and wiring to main; his code otherwise as he wrote it.
• [ ] Nothing imports src/display/launch.ts, src/arcade/host.ts or src/arcade/snapshot.ts yet (they are the next issue).
• [ ] A display.json is parsed and validated at the boundary: a test reads a good one 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: src/display/config.ts, src/display/items.ts, src/display/known.ts, src/display/selections.ts, src/arcade/shelf.ts, test/display-model.test.ts. Risks: needs src/arcade/carts.ts (#9197) and src/actions/pin_demos.ts (#9194) on main first; do not touch src/http/api.ts.

Parked by the crew: the run ended without a pull: declined: the prerequisites this issue names are not on main yet, so the display model cannot compile there.

Parked by the crew: the run ended without a pull: declined: the display model can't compile on main yet. bun x tsc --noEmit fails on the five staged files because the code they import isn't on main.

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the run ended without a pull: declined: the prerequisites this issue names are not on main yet, so the display model cannot compile there
2. the run ended without a pull: declined: the display model can't compile on main yet. bun x tsc --noEmit fails on the five staged files because the code they import isn't on main

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split: parked twice because what it imports is not on main. src/actions/pin_demos.ts and launchOf in src/git/builds.ts (#9194 was closed without them) are now #9272. The four src/core/repos.ts reads (listAdminOwnedRepos, readableRepoIds, repoDatesFor, listAllReposByOwner) are now #9273. The model itself is #9274, which waits on both. #9208 now waits on #9274.

Correction: the forge dropped #9274 as a duplicate because its title matched this one. The model is #9279, which waits on #9272 and #9273. #9208 now waits on #9279. Closed as split into #9272, #9273 and #9279.