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/testdata/request_user_games_order.json

@@ -1,13 +1,13 @@
 {
-  "message_type": "user.games.command",
+  "message_type": "user.games.order",
   "protocol_version": "v1",
   "device_session_id": "device-session-1",
   "timestamp_ms": 1700000001000,
   "request_id": "req-games-1",
   "payload": "games-payload",
   "payload_hash_hex": "a8322c99bf424939cd3a1e5a41b5edb67e567bff87c49e8ff229be60976960e0",
-  "expected_canonical_bytes_hex": "1167616c6178792d726571756573742d7631027631106465766963652d73657373696f6e2d3112757365722e67616d65732e636f6d6d616e640000018bcfe56be80b7265712d67616d65732d3120a8322c99bf424939cd3a1e5a41b5edb67e567bff87c49e8ff229be60976960e0",
+  "expected_canonical_bytes_hex": "1167616c6178792d726571756573742d7631027631106465766963652d73657373696f6e2d3110757365722e67616d65732e6f726465720000018bcfe56be80b7265712d67616d65732d3120a8322c99bf424939cd3a1e5a41b5edb67e567bff87c49e8ff229be60976960e0",
   "private_key_seed_hex": "0303030303030303030303030303030303030303030303030303030303030303",
   "public_key_base64": "7UkoxijRwsbq6QM4kFmVYSlZJzpcY/k2NsFGFKyHN9E=",
-  "expected_signature_hex": "262d5480451560d9b2ca96468b0e962e4288eabb4dff29dbc66c491a37dd92b779d2b89853083a695317f8535e49c402dcfd49a11fd2926f3af42ceb745e2b0a"
+  "expected_signature_hex": "b4a9a1eb2eb67777469d87ed7c513afd89f0c0a96ca9e0d945cd327830c11a7834ac073f6a872d8121cd7674b9491399a0256210292615bd424d11d0db37200c"
 }