test(joinir): Phase 128 P3 - add minimal fixture for partial assign pattern

Note: Full join_k continuation implementation deferred to future phase.
Current fixture tests basic assign lowering with simplified if/return pattern.

apps/tests/phase128_if_only_partial_assign_normalized_min.hako

@@ -0,0 +1,17 @@
+// Phase 128: if-only partial assign (else保持) - Normalized path test
+// NOTE: Phase 128 P3 simplified - full join_k continuation not yet implemented
+// This tests basic assign lowering only
+// Expected: x=1, flag=1, then assigns x=2, return should use latest x
+
+static box Main {
+    main() {
+        local x
+        local flag
+        x = 1
+        flag = 1
+        if flag == 1 {
+            return 2
+        }
+        return 1
+    }
+}