Run an Everruns serve app durably on celld

The new everruns-serve-celld hosting target runs a serve app on celld. The serve binary exposes /celld/state, /celld/snapshot and /celld/restore, and boots lazily so a restore always comes first.

The celld example adds the app, a Dockerfile and the celld worker: a Durable Object per name that journals mutating requests, snapshots the data directory when idle, and restores and replays when its container is fresh. An end-to-end test kills serve mid-turn, and then the node itself.

See Serve on celld. PR #4019.