feat(api): promote Factor/Schedule/VarStore to pub in factors module

Exposes the factor-graph machinery so power users can define custom
factors and schedules (see Game::custom in the next task). The
internal factor/ and schedule/ modules remain unchanged (still
referenced by Game's internals via crate::factor); the user-facing
public API goes through the new factors module re-exports:

  pub use crate::factor::{BuiltinFactor, Factor, VarId, VarStore};
  pub use crate::factor::rank_diff::RankDiffFactor;
  pub use crate::factor::team_sum::TeamSumFactor;
  pub use crate::factor::trunc::TruncFactor;
  pub use crate::schedule::{EpsilonOrMax, Schedule, ScheduleReport};

#[allow(dead_code)] guards on the previously-pub(crate) items are
removed because the types are now referenced via the re-exports.

Promotes public methods on VarStore (len, alloc, get, set, clear, new)
and adds is_empty per clippy lint. Keeps marginals field private as an
implementation detail — users access via the public methods.

Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/factor/trunc.rs

@@ -11,10 +11,10 @@ use crate::{
 /// Stores its outgoing message to the diff variable so the cavity computation
 /// produces the correct EP message on each propagation.
 #[derive(Debug)]
-pub(crate) struct TruncFactor {
-    pub(crate) diff: VarId,
-    pub(crate) margin: f64,
-    pub(crate) tie: bool,
+pub struct TruncFactor {
+    pub diff: VarId,
+    pub margin: f64,
+    pub tie: bool,
     /// Outgoing message to the diff variable (initial: N_INF, the EP identity).
     pub(crate) msg: Gaussian,
     /// Cached evidence (linear, not log) computed from the cavity on first propagation.
@@ -22,7 +22,7 @@ pub(crate) struct TruncFactor {
 }
 
 impl TruncFactor {
-    pub(crate) fn new(diff: VarId, margin: f64, tie: bool) -> Self {
+    pub fn new(diff: VarId, margin: f64, tie: bool) -> Self {
         Self {
             diff,
             margin,