TerrainMod

Code cleanup and performance improvements
Cached the main terrain collider after the first lookup instead of repeatedly searching for the terrain object during normal update/aiming behavior.
Reset the cached terrain reference when the active world/save state is cleared, preventing stale terrain references between sessions.
Replaced unnecessary reflection for standard Unity TerrainData properties with direct property access.
Updated brush preview logic to use direct TerrainData access for alphamap dimensions, terrain size, and height sampling.
Updated aiming and painting coordinate calculations to use direct TerrainData properties.
Replaced reflected GetInterpolatedHeight calls with direct TerrainData.GetInterpolatedHeight(...) calls.
Updated pending save replay logic to use the cached terrain lookup path.
Kept the lower-level alphamap mutation and restore logic unchanged where IL2CPP-specific handling is still needed.