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/full/configs/hosts.yaml

@@ -0,0 +1,39 @@
+---
+hosts:
+  - hostname: default
+    type: "x86_64-linux-gnu"
+    model: "x86_64-linux-gnu"
+    max_slot: -1
+    description: "default host"
+    resource:
+      - name: mg
+      - name: maxswap
+        value: 10000
+    sched_stop:
+      - resname: r1m
+        sched: 3.5
+        stop: 4.5
+      - resname: r15s
+        sched: 4.0
+        stop: 10.0
+    dispatch_window:
+      - start_time: "9:00"
+        end_time: "18:00"
+      - start_time: "21:00"
+        end_time: "7:00"
+
+  - hostname: worker2
+    model: "!"
+    type: "!"
+    max_slot: 256
+    resource:
+      - name: gstring
+        value: "This is a G string"
+
+
+hostgroups:
+  - groupname: hg1
+    members:
+      - all
+    exclude_members: [h3]
+