Fix TSTMstStgs settings callback leak

[appaKappaK]

Summary

Fixes #82

Fixes a KSPCF-reported memory leak where destroyed TSTMstStgs instances remain subscribed to GameEvents.OnGameSettingsApplied.

Root Cause

TSTMstStgs.Start() subscribes ApplySettings to GameEvents.OnGameSettingsApplied, but OnDestroy() was adding the callback again instead of removing it. Since TSTMstStgs is created for every scene, destroyed instances could accumulate as event callback owners across scene changes.

Changes

• Unsubscribe ApplySettings from GameEvents.OnGameSettingsApplied in OnDestroy()
• Clear TSTMstStgs.Instance when the destroyed object is the active singleton

Validation

Built and tested locally in KSP. After scene changes and roughly 2 hours of play, the previous KSPCF TarsierSpaceTech:TSTMstStgs / OnGameSettingsApplied warning did not reappear.