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/competitor.rs

@@ -30,7 +30,7 @@ impl<T: Time, D: Drift<T>> Competitor<T, D> {
         match self.message {
             Some(message) => {
                 let elapsed_variance = match &self.last_time {
-                    Some(last) => self.rating.drift.variance_delta(last, now),
+                    Some(last) => self.rating.drift_variance_delta(last, now),
                     None => 0.0,
                 };
 
@@ -46,7 +46,7 @@ impl<T: Time, D: Drift<T>> Competitor<T, D> {
     /// and should not be recomputed from `last_time` (which may have shifted).
     pub(crate) fn receive_for_elapsed(&self, elapsed: i64) -> Gaussian {
         match self.message {
-            Some(message) => message.forget(self.rating.drift.variance_for_elapsed(elapsed)),
+            Some(message) => message.forget(self.rating.drift_variance_for_elapsed(elapsed)),
             None => self.rating.prior,
         }
     }