Edge protocol versions: old store builds keep working or ask to update #2878

Part of #2875. Store builds stay installed for months, so the edge API they call must not break under them.

• Each app's socket hello and JSON API carry a protocol version; the edge answers with the oldest version it still serves.
• A client below that floor shows "Update to keep playing" with a store link instead of failing.
• A guard in @rustybuns/util (shared by edge and page) for the hello, and a test that an old-version client gets the update reply.
• Start with Park Hide & Seek's /ws (the iOS pilot), then Hippo Tycoon.
• The stage these builds call must never be torn down in a cost sweep: note it in COSTS.md and the live-deploys list.