refactor: make BuiltinFactor::log_evidence match exhaustive

Replace the `_ => 0.0` wildcard with explicit
`Self::TeamSum(_) | Self::RankDiff(_) => 0.0`. No behavioral change;
future variants now produce a compile error instead of being silently
absorbed by the wildcard.

src/factor/mod.rs

@@ -95,7 +95,7 @@ impl Factor for BuiltinFactor {
         match self {
             Self::Trunc(f) => f.log_evidence(vars),
             Self::Margin(f) => f.log_evidence(vars),
-            _ => 0.0,
+            Self::TeamSum(_) | Self::RankDiff(_) => 0.0,
         }
     }
 }