fix: correct _majorVersion() to extract full major version component

[maradini77]

Summary

Fixes a bug in SemVerMixin._majorVersion() that was returning only the first byte of the version string (e.g., 'v' from "v1.2.3") instead of the complete major version component (e.g., "1"). This caused the EIP-712 domain separator to remain unchanged across major version upgrades, violating signature isolation between incompatible releases.

Problem

The previous implementation of _majorVersion() returned only the first character:

return string(abi.encodePacked(v[0])); // Returns 'v' or first digit

This meant:

• "v1.2.3" and "v2.0.0" would both contribute the same value ('v') to the domain separator
• EIP-712 signatures from older major versions remained valid after contract upgrades
• The intended security property of signature invalidation on major version changes was not enforced

Solution

Updated _majorVersion() to:

1. Skip optional 'v'/'V' prefix
2. Extract all characters until the first '.'
3. Return the complete major version component as a string

This ensures the domain separator changes correctly when the major version is incremented, properly isolating signatures across incompatible releases.