fix: Skip read-only properties in mock resolution

[jneb802]

Summary

• Adds HasSetMethod property to MemberBase to track whether a member has a setter
• Guards SetValue calls in MockManager.FixMemberReferences with HasSetMethod check, preventing crashes on read-only properties like Component.transform

Problem

When mods trigger FixReferences on GameObjects with read-only component properties (e.g. Component.transform), MockManager.FixMemberReferences crashes with:

ArgumentException: Set Method not found for 'transform'
  at System.Reflection.RuntimePropertyInfo.SetValue(...)
  at Jotunn.PropertyMember.SetValue(...)
  at Jotunn.Managers.MockManager.FixMemberReferences(...)

The method iterates all properties via reflection and calls SetValue without checking if a setter exists.

Steps to reproduce

1. Install Jotunn v2.27.1, ExpandWorldData, and More World Locations AIO
2. Launch the game and load into a world
3. During ZoneSystem.Start, EWD triggers SoftReference.Load() on MWL location prefabs
4. Jotunn's AssetManager.SwapResolvedAsset hook fires, calling PrefabExtension.FixReferences on the loaded prefab
5. MockManager.FixMemberReferences encounters read-only properties like Component.transform and crashes calling SetValue

Fix

Follows the existing HasGetMethod pattern — adds a symmetric HasSetMethod property and checks it before SetValue calls in the two branches that write values back.