feat: allow drift to vary per competitor via Member::with_drift_scale

Drift was a property of the History, so every competitor drifted at the
same rate and a fixed reference point could not share a graph with moving
competitors. A bot at a known strength, a rating floor, a course
difficulty — all of them drifted along with the players.

Member::with_drift_scale(s) multiplies the drift *variance* a competitor
accumulates, so s is in the same units as gamma: ConstantDrift(g) at
scale s behaves exactly as ConstantDrift(g * s) would for that competitor.
A scalar rather than a per-competitor Drift keeps History's single D type
parameter untouched and stays Copy. 0.0 pins a competitor still.

The scale lives on Rating, beside the drift it scales, and is applied
only through Rating::drift_variance_delta / drift_variance_for_elapsed.
Making those the sole entry points means a caller cannot reach the raw
drift and silently skip a competitor's scale — the filtered pass was
exactly that bug during development, caught because its test was written
before the wiring.

Like with_prior, the scale is competitor configuration captured at first
appearance rather than a per-event override; a competitor that is static
is static, and a scale that changed between events would make the skill
trajectory hard to interpret. Member's docs claimed prior was a per-event
override, which the code has never done — corrected here.

A negative scale is rejected rather than squared into its absolute value,
and a non-finite one rejected outright, both as InvalidParameter.

None means 1.0, so no existing call site changes and no existing fit
moves. Adding a public field to Member does break struct-literal
construction downstream.

Closes #34

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014b6wy2q8rnFK8U8GPJVQNU

src/time_slice.rs

@@ -72,9 +72,10 @@ impl Item {
         let skill = skills.at(self.slot);
 
         if forward {
-            Rating::new(skill.forward, r.beta, r.drift)
+            Rating::new(skill.forward, r.beta, r.drift).with_drift_scale(r.drift_scale)
         } else {
             Rating::new(skill.posterior() / self.likelihood, r.beta, r.drift)
+                .with_drift_scale(r.drift_scale)
         }
     }
 }
@@ -589,8 +590,7 @@ impl<T: Time> TimeSlice<T> {
         n.forget(
             agents[*agent]
                 .rating
-                .drift
-                .variance_for_elapsed(skill.elapsed),
+                .drift_variance_for_elapsed(skill.elapsed),
         )
     }
 
@@ -645,7 +645,7 @@ impl<T: Time> TimeSlice<T> {
             let rating = &agents[agent].rating;
 
             let forward = match incoming.get(&agent) {
-                Some(message) => message.forget(rating.drift.variance_for_elapsed(skill.elapsed)),
+                Some(message) => message.forget(rating.drift_variance_for_elapsed(skill.elapsed)),
                 None => rating.prior,
             };