List erf/erfinv in the README's math vocabulary

They shipped without ever being added to the public surface list.
Scalar-only is noted explicitly: every other scalar in math.cs has
float2/3/4 overloads, erf/erfinv do not, and that stays a separate
scope decision since the only consumer (Aetheria fire control) is
scalar.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/cultmath/README.md

@@ -26,7 +26,8 @@ shader semantics.
   acceleration and semi-implicit Euler integration
 - `math` intrinsics: `radians`, `degrees`, `abs`, `floor`, `ceil`, `frac`,
   `min`, `max`, `clamp`, `saturate`, `lerp`, `step`, `smoothstep`, `dot`,
-  `cross`, `length`, `distance`, `normalize`, `reflect`, `csum`, exponential
+  `cross`, `length`, `distance`, `normalize`, `reflect`, `csum`, `erf`,
+  `erfinv` (scalar only), exponential
   `decay`/`damp`, intercept/segment helpers, Catmull-Rom, Bezier curves, and
   deterministic value-noise primitives
 - `shaders/CultMath.hlsl`, a canonical HLSL mirror include for shader-side