The wrenDefineMethod and wrenDefineStaticMethod functions now accept a single `signature` string parameter instead of separate `methodName` and `arity` arguments. This simplifies the API by letting callers specify the full method signature (including arity spaces) directly, removing the internal arity-to-spaces conversion logic in defineMethod. The change updates the header declaration, the internal defineMethod helper, and all call sites in wren_io.c to pass pre-formatted signatures.