feat: introduce InferenceError; mu_sigma panic already eliminated

mu_sigma was deleted as part of the Gaussian nat-param rewrite (its
only callers were the old Mul/Div impls). This commit adds the
InferenceError enum as a seed for the T2 API surface, with the
NegativePrecision variant that mu_sigma would have returned.

Part of T0 engine redesign.

src/error.rs

@@ -0,0 +1,18 @@
+use std::fmt;
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum InferenceError {
+    NegativePrecision { pi: f64 },
+}
+
+impl fmt::Display for InferenceError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NegativePrecision { pi } => {
+                write!(f, "precision must be non-negative; got {pi}")
+            }
+        }
+    }
+}
+
+impl std::error::Error for InferenceError {}

src/lib.rs

@@ -11,6 +11,7 @@ pub mod agent;
 mod approx;
 pub mod batch;
 pub mod drift;
+mod error;
 mod game;
 pub mod gaussian;
 mod history;
@@ -19,6 +20,7 @@ mod message;
 pub mod player;
 
 pub use drift::{ConstantDrift, Drift};
+pub use error::InferenceError;
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;