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/openapi.yaml

@@ -436,21 +436,24 @@ components:
     CommandRequest:
       type: object
       description: |
-        Batch command payload. `actor` identifies the race submitting the commands.
-        Each element of `cmd` is a polymorphic command object discriminated by the
-        `@type` field. At least one command is required.
+        Order payload for `PUT /api/v1/order`. `actor` identifies the race
+        submitting the order. Each element of `cmd` is a polymorphic command
+        object discriminated by the `@type` field. An empty `cmd` array is
+        valid: it clears the player's stored order for the turn (equivalent to
+        removing every command).
       required:
         - actor
         - cmd
       properties:
         actor:
           type: string
-          description: Race name of the actor submitting the commands. Must be non-blank.
+          description: Race name of the actor submitting the order. Must be non-blank.
           minLength: 1
         cmd:
           type: array
-          description: One or more commands to execute in order.
-          minItems: 1
+          description: >-
+            The player's commands, applied in submission order at turn
+            generation. May be empty to clear the stored order.
           items:
             $ref: "#/components/schemas/Command"
     UserGamesOrder: