chore(cleanup): purge /command residuals — fakeEngine, canon golden, openapi

Follow-up tidy after the cross-service /command removal (#73):

- Rename the router test double dummyExecutor -> fakeEngine (and the
  newExecutor / setupRouterExecutor helpers -> newFakeEngine /
  setupRouterEngine): it implements handler.Engine now, "executor" was a
  leftover of the removed adapter. Test-only.

- Regenerate the ui/core canon signing golden onto user.games.order
  (request_user_games_command.json -> request_user_games_order.json, fresh
  canonical bytes + Ed25519 signature) and drop the last
  user.games.command references from the Go/TS tests and docs.

- Align game openapi: CommandRequest.cmd no longer carries minItems: 1. It
  is now used only by PUT /api/v1/order, which accepts an empty batch
  (clearing the player's stored order, equivalent to removing every
  command); the contract test freezes the empty-allowed shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ui/PLAN.md

@@ -469,7 +469,7 @@ Acceptance criteria:
 
 - canonical-bytes output matches gateway-side output byte-for-byte
   for the three Phase-3 message types (`user.account.get`,
-  `lobby.my.games.list`, `user.games.command`);
+  `lobby.my.games.list`, `user.games.order`);
 - a request signed by `ui/core` is accepted by the gateway's own
   verifier in a unit test (`TestParityRequestSignedByUICoreAcceptedByGateway`);
 - a response signed by `gateway/authn`'s `Ed25519ResponseSigner` is
@@ -648,7 +648,7 @@ Acceptance criteria (met):
   KB measured);
 - `WasmCore.signRequest` produces canonical bytes byte-for-byte
   identical to the gateway-side fixtures for three message types
-  (`request_user_account_get`, `request_user_games_command`,
+  (`request_user_account_get`, `request_user_games_order`,
   `request_lobby_my_games_list`);
 - `WasmCore` exposes the same `Core` TypeScript types future
   `WailsCore` and `CapacitorCore` adapters will satisfy.