feat(factor): introduce Factor trait and BuiltinFactor enum

Adds the trait that all factors implement and the enum dispatcher
used by the schedule to drive heterogeneous factors without dynamic
dispatch in the hot loop.

The three built-in factors (TeamSum, RankDiff, Trunc) are stubbed
out; concrete implementations follow in tasks 4-6.

src/factor/team_sum.rs

@@ -0,0 +1,16 @@
+use crate::{
+    factor::{Factor, VarId, VarStore},
+    gaussian::Gaussian,
+};
+
+#[derive(Debug)]
+pub(crate) struct TeamSumFactor {
+    pub(crate) inputs: Vec<(Gaussian, f64)>,
+    pub(crate) out: VarId,
+}
+
+impl Factor for TeamSumFactor {
+    fn propagate(&mut self, _vars: &mut VarStore) -> (f64, f64) {
+        unimplemented!("TeamSumFactor stub — implemented in Task 4")
+    }
+}