fix(depgraph): detect a measure that references itself as a cycle

Symptom: a model containing 'M1 = [M1] + 1' passed the cycle check and the gate
reported no circular dependency, while the real engine refuses to refresh it.

Root cause: add_edge dropped any edge whose endpoints were equal, so the self loop
never reached the Tarjan pass and the single node component never triggered the self
edge branch. Self edges are now stored.

Author: sandeep0430

src/semantic_diff/depgraph.py

@@ -60,8 +60,8 @@ def add_node(self, node_id: str, kind: str, label: str, detail: str = "") -> Non
             self.reverse.setdefault(node_id, set())
 
     def add_edge(self, dependent: str, dependency: str) -> None:
-        if dependent == dependency:
-            return
+        # Self edges are kept. A measure whose expression references itself is a real
+        # cycle that fails refresh, and dropping the edge made find_cycles blind to it.
         self.edges.setdefault(dependent, set()).add(dependency)
         self.reverse.setdefault(dependency, set()).add(dependent)
         self.edges.setdefault(dependency, set())