feat(api): add Send + Sync bounds to public traits

Required for T3 rayon-based parallelism. Affected traits:
- Time (+ Send + Sync + 'static)
- Drift<T> (+ Send + Sync)
- Observer<T> (+ Send + Sync)
- Factor (+ Send + Sync)
- Schedule (+ Send + Sync)

All built-in impls (i64, Untimed, ConstantDrift, NullObserver,
EpsilonOrMax, TeamSumFactor, RankDiffFactor, TruncFactor,
BuiltinFactor) naturally satisfy these bounds via auto-derive.

Minor breaking change: downstream custom impls that aren't already
thread-safe will need to add the bounds.

Part of T3 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/drift.rs

@@ -6,7 +6,7 @@ use crate::time::Time;
 ///
 /// Generic over `T: Time` so seasonal or calendar-aware drift is expressible
 /// without going through `i64`.
-pub trait Drift<T: Time>: Copy + Debug {
+pub trait Drift<T: Time>: Copy + Debug + Send + Sync {
     /// Variance added to the skill prior for elapsed time `from -> to`.
     ///
     /// Called with `from <= to`; returning zero means no drift accumulates.

src/factor/mod.rs

@@ -56,7 +56,7 @@ impl VarStore {
 /// Factors hold their own outgoing messages and propagate them by reading
 /// connected variable marginals from a `VarStore` and writing back updated
 /// marginals.
-pub trait Factor {
+pub trait Factor: Send + Sync {
     /// Update outgoing messages and write back to the var store.
     ///
     /// Returns the max delta `(|Δmu|, |Δsigma|)` across writes this

src/observer.rs

@@ -9,9 +9,8 @@ use crate::time::Time;
 /// Receives progress callbacks during `History::converge`.
 ///
 /// All methods have default no-op implementations; implement only what's
-/// interesting. Send/Sync is NOT required in T2 (added in T3 along with
+/// interesting.
-/// Rayon support).
+pub trait Observer<T: Time>: Send + Sync {
-pub trait Observer<T: Time> {
     /// Called after each convergence iteration across the whole history.
     fn on_iteration_end(&self, _iter: usize, _max_step: (f64, f64)) {}
 

src/schedule.rs

@@ -16,7 +16,7 @@ pub struct ScheduleReport {
 }
 
 /// Drives factor propagation to convergence.
-pub trait Schedule {
+pub trait Schedule: Send + Sync {
     fn run(&self, factors: &mut [BuiltinFactor], vars: &mut VarStore) -> ScheduleReport;
 }
 

src/time.rs

@@ -8,7 +8,7 @@
 ///
 /// Must be `Ord + Copy` so slices can sort events, and `'static` so
 /// `History` can store it by value without lifetimes.
-pub trait Time: Copy + Ord + 'static {
+pub trait Time: Copy + Ord + Send + Sync + 'static {
     /// How much time elapsed between `self` and `later`.
     ///
     /// Used by `Drift<T>::variance_delta` to compute skill drift. Returning