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/battle_test.go

@@ -29,8 +29,8 @@ func TestGetBattleValidation(t *testing.T) {
 		{"Invalid uuid", "0", invalidId, http.StatusBadRequest},
 	} {
 		t.Run(tc.description, func(t *testing.T) {
-			e := &dummyExecutor{}
-			r := setupRouterExecutor(e)
+			e := &fakeEngine{}
+			r := setupRouterEngine(e)
 
 			w := httptest.NewRecorder()
 			path := fmt.Sprintf("/api/v1/battle/%s/%s", tc.turn, tc.battleID)
@@ -81,11 +81,11 @@ func TestGetBattleFound(t *testing.T) {
 			{Attacker: 0, AttackerShipClass: 10, Defender: 1, DefenderShipClass: 20, Destroyed: true},
 		},
 	}
-	e := &dummyExecutor{
+	e := &fakeEngine{
 		FetchBattleResult: stored,
 		FetchBattleOK:     true,
 	}
-	r := setupRouterExecutor(e)
+	r := setupRouterEngine(e)
 
 	w := httptest.NewRecorder()
 	path := fmt.Sprintf("/api/v1/battle/%d/%s", 7, id.String())
@@ -111,11 +111,11 @@ func TestGetBattleFound(t *testing.T) {
 
 func TestGetBattleTurnZero(t *testing.T) {
 	id := uuid.New()
-	e := &dummyExecutor{
+	e := &fakeEngine{
 		FetchBattleResult: &report.BattleReport{ID: id},
 		FetchBattleOK:     true,
 	}
-	r := setupRouterExecutor(e)
+	r := setupRouterEngine(e)
 
 	w := httptest.NewRecorder()
 	req, _ := http.NewRequest(http.MethodGet, fmt.Sprintf("/api/v1/battle/0/%s", id.String()), nil)
@@ -128,8 +128,8 @@ func TestGetBattleTurnZero(t *testing.T) {
 
 func TestGetBattleNotFound(t *testing.T) {
 	id := uuid.New()
-	e := &dummyExecutor{FetchBattleOK: false}
-	r := setupRouterExecutor(e)
+	e := &fakeEngine{FetchBattleOK: false}
+	r := setupRouterEngine(e)
 
 	w := httptest.NewRecorder()
 	req, _ := http.NewRequest(http.MethodGet, fmt.Sprintf("/api/v1/battle/3/%s", id.String()), nil)
@@ -141,8 +141,8 @@ func TestGetBattleNotFound(t *testing.T) {
 }
 
 func TestGetBattleEngineError(t *testing.T) {
-	e := &dummyExecutor{FetchBattleErr: errors.New("engine boom")}
-	r := setupRouterExecutor(e)
+	e := &fakeEngine{FetchBattleErr: errors.New("engine boom")}
+	r := setupRouterEngine(e)
 
 	w := httptest.NewRecorder()
 	req, _ := http.NewRequest(http.MethodGet, fmt.Sprintf("/api/v1/battle/3/%s", uuid.NewString()), nil)