add WIDTH constant to Poseidon2 types for compile-time safety

[chetanyb]

Summary

Adds a WIDTH constant to Poseidon2 type definitions to enable compile-time width validation.

Motivation

Poseidon2 has multiple width variants, but there was no programmatic way to verify which variant a type represents at compile time. This can lead to subtle bugs where code expects one width but receives another.

Benefits

1. Compile-time safety: Downstream code can verify the width with:

   comptime {
       if (Poseidon2Type.WIDTH != 24) {
           @compileError("Expected width-24");
       }
   }
   

2. Better type documentation: The width is now part of the type's public interface
3. Prevents runtime errors: Catches width mismatches before code runs

[ch4r10t33r]

LGTM