test(simulation): assert "mixed" faction → ContestedZone (#956)

PR #152 review (Hoshe): the faction→TerritorialStatus mapping test covered 7 of
the 8 D-237 values; add the missing "mixed" assertion (grouped with "disputed",
behavior already correct).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/attractor_matching.rs

@@ -877,6 +877,10 @@ mod tests {
             territorial_status_from_faction(Some("disputed")),
             TerritorialStatus::ContestedZone
         );
+        assert_eq!(
+            territorial_status_from_faction(Some("mixed")),
+            TerritorialStatus::ContestedZone
+        );
         assert_eq!(
             territorial_status_from_faction(None),
             TerritorialStatus::FrontierUnclaimed