feat(factor): introduce VarId and VarStore

Foundation types for the T1 factor graph machinery. VarStore is a
flat Vec<Gaussian> indexed by VarId; variables are allocated by
alloc() and the store can be cleared between games to reuse capacity.

Part of T1 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/factor/mod.rs

@@ -0,0 +1,94 @@
+//! Factor graph machinery for within-game inference.
+
+use crate::gaussian::Gaussian;
+
+/// Identifier for a variable in a `VarStore`.
+///
+/// Variables hold the current Gaussian marginal and are owned by exactly one
+/// `VarStore`. `VarId` is meaningful only within its owning store.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub(crate) struct VarId(pub(crate) u32);
+
+/// Flat storage of variable marginals.
+///
+/// Variables are allocated by `alloc()` and accessed by `VarId`. The store is
+/// reused across `Game::new` calls (it lives in the `ScratchArena`); call
+/// `clear()` before reuse.
+#[derive(Debug, Default)]
+pub(crate) struct VarStore {
+    pub(crate) marginals: Vec<Gaussian>,
+}
+
+impl VarStore {
+    pub(crate) fn new() -> Self {
+        Self::default()
+    }
+
+    pub(crate) fn clear(&mut self) {
+        self.marginals.clear();
+    }
+
+    pub(crate) fn len(&self) -> usize {
+        self.marginals.len()
+    }
+
+    pub(crate) fn alloc(&mut self, init: Gaussian) -> VarId {
+        let id = VarId(self.marginals.len() as u32);
+        self.marginals.push(init);
+        id
+    }
+
+    pub(crate) fn get(&self, id: VarId) -> Gaussian {
+        self.marginals[id.0 as usize]
+    }
+
+    pub(crate) fn set(&mut self, id: VarId, g: Gaussian) {
+        self.marginals[id.0 as usize] = g;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::N_INF;
+
+    #[test]
+    fn alloc_assigns_sequential_ids() {
+        let mut store = VarStore::new();
+        let a = store.alloc(N_INF);
+        let b = store.alloc(N_INF);
+        let c = store.alloc(N_INF);
+        assert_eq!(a, VarId(0));
+        assert_eq!(b, VarId(1));
+        assert_eq!(c, VarId(2));
+        assert_eq!(store.len(), 3);
+    }
+
+    #[test]
+    fn get_returns_initial_value() {
+        let mut store = VarStore::new();
+        let g = Gaussian::from_ms(2.5, 1.0);
+        let id = store.alloc(g);
+        assert_eq!(store.get(id), g);
+    }
+
+    #[test]
+    fn set_updates_value() {
+        let mut store = VarStore::new();
+        let id = store.alloc(N_INF);
+        let new = Gaussian::from_ms(3.0, 0.5);
+        store.set(id, new);
+        assert_eq!(store.get(id), new);
+    }
+
+    #[test]
+    fn clear_resets_length_keeping_capacity() {
+        let mut store = VarStore::new();
+        store.alloc(N_INF);
+        store.alloc(N_INF);
+        let cap = store.marginals.capacity();
+        store.clear();
+        assert_eq!(store.len(), 0);
+        assert_eq!(store.marginals.capacity(), cap);
+    }
+}

src/lib.rs

@@ -13,6 +13,7 @@ pub(crate) mod arena;
 pub mod batch;
 pub mod drift;
 mod error;
+pub(crate) mod factor;
 mod game;
 pub mod gaussian;
 mod history;