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/core/canon/signature_test.go

@@ -22,7 +22,7 @@ func TestVerifyRequestSignature(t *testing.T) {
 	fields := canon.RequestSigningFields{
 		ProtocolVersion: "v1",
 		DeviceSessionID: "device-session-123",
-		MessageType:     "user.games.command",
+		MessageType:     "user.games.order",
 		TimestampMS:     123456789,
 		RequestID:       "request-123",
 		PayloadHash:     sha256Sum([]byte("payload")),