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>

game/internal/router/router_helper_test.go

@@ -31,7 +31,7 @@ func id() string {
 	return uuid.New().String()
 }
 
-type dummyExecutor struct {
+type fakeEngine struct {
 	CommandsExecuted int
 
 	// ValidateOrderResult, when non-nil, is returned from ValidateOrder.
@@ -55,7 +55,7 @@ type dummyExecutor struct {
 	FetchBattleErr    error
 }
 
-func (e *dummyExecutor) ValidateOrder(actor string, cmd ...order.DecodableCommand) (*order.UserGamesOrder, error) {
+func (e *fakeEngine) ValidateOrder(actor string, cmd ...order.DecodableCommand) (*order.UserGamesOrder, error) {
 	e.CommandsExecuted = len(cmd)
 	if e.ValidateOrderErr != nil {
 		return nil, e.ValidateOrderErr
@@ -70,48 +70,48 @@ func (e *dummyExecutor) ValidateOrder(actor string, cmd ...order.DecodableComman
 	}, nil
 }
 
-func (e *dummyExecutor) FetchOrder(actor string, turn uint) (*order.UserGamesOrder, bool, error) {
+func (e *fakeEngine) FetchOrder(actor string, turn uint) (*order.UserGamesOrder, bool, error) {
 	e.FetchOrderActor = actor
 	e.FetchOrderTurn = turn
 	return e.FetchOrderResult, e.FetchOrderOK, e.FetchOrderErr
 }
 
-func (e *dummyExecutor) FetchBattle(turn uint, ID uuid.UUID) (*report.BattleReport, bool, error) {
+func (e *fakeEngine) FetchBattle(turn uint, ID uuid.UUID) (*report.BattleReport, bool, error) {
 	e.FetchBattleTurn = turn
 	e.FetchBattleID = ID
 	return e.FetchBattleResult, e.FetchBattleOK, e.FetchBattleErr
 }
 
-func (e *dummyExecutor) GenerateGame(gameID uuid.UUID, races []string) (game.State, error) {
+func (e *fakeEngine) GenerateGame(gameID uuid.UUID, races []string) (game.State, error) {
 	return game.State{ID: gameID}, nil
 }
 
-func (e *dummyExecutor) GenerateTurn() (game.State, error) {
+func (e *fakeEngine) GenerateTurn() (game.State, error) {
 	return game.State{}, nil
 }
 
-func (e *dummyExecutor) BanishRace(raceName string) error {
+func (e *fakeEngine) BanishRace(raceName string) error {
 	return nil
 }
 
-func (e *dummyExecutor) GameState() (game.State, error) {
+func (e *fakeEngine) GameState() (game.State, error) {
 	return game.State{}, nil
 }
 
-func (e *dummyExecutor) LoadReport(actor string, turn uint) (*report.Report, error) {
+func (e *fakeEngine) LoadReport(actor string, turn uint) (*report.Report, error) {
 	return &report.Report{}, nil
 }
 
 func setupRouter() *gin.Engine {
-	return setupRouterExecutor(newExecutor())
+	return setupRouterEngine(newFakeEngine())
 }
 
-func setupRouterExecutor(e handler.Engine) *gin.Engine {
+func setupRouterEngine(e handler.Engine) *gin.Engine {
 	return router.SetupRouter(e)
 }
 
-func newExecutor() handler.Engine {
-	return &dummyExecutor{}
+func newFakeEngine() handler.Engine {
+	return &fakeEngine{}
 }
 
 // newService builds a real controller.Service backed by a storage directory,