feat(admin-config-tool): task 5.4 - golden-file build determinism tests

tests/fixtures/{minimal,full}/configs/*.yaml + checked-in expected.bin
golden files, added to test_config_build.py. Catches representation
drift (bytes changing even when decoded content stays correct) that
the existing structural/decode-based assertions can't detect.

src/xceadmin/tests/fixtures/minimal/configs/hosts.yaml

@@ -0,0 +1,8 @@
+hosts:
+  - hostname: worker1
+    type: x86_64-linux-gnu
+    max_slot: 8
+hostgroups:
+  - groupname: all_hosts
+    members:
+      - worker1

src/xceadmin/tests/fixtures/minimal/configs/queues.yaml

@@ -0,0 +1,3 @@
+queues:
+  - name: normal
+    priority: 30