refactor: make StateCache descriptor-free

tests/test_state_cache.py

@@ -0,0 +1,15 @@
+from samsung_appliance.state_cache import StateCache
+
+
+def test_state_cache_constructs_without_descriptor():
+    c = StateCache()
+    assert c.links == {}
+    assert c.descriptor_state == {}
+
+
+def test_on_observation_hook_fires():
+    c = StateCache()
+    seen = []
+    c.set_on_observation(lambda st, href, rep: seen.append((href, rep)))
+    c.apply_rep('/x/vs/0', {'a': 1}, source='seed')
+    assert seen == [('/x/vs/0', {'a': 1})]