Fix Overlapping TTS Playback when different characters are speaking in turns

[JoeyBelvar]

Intent: stop outgoing NPC dialogue audio when the talk line changes, especially when skipping from one speaker to another.

• Stops the active NPC speech object before _currentSpeechObject is replaced by the next speaker.
• Keeps existing request cancellation behavior so stale relay responses cannot play after dialogue advances.
• Wires Core media diagnostics into the plugin log under the existing [NPC TTS] logging style.
• Adds maintainer comments around the turn-swap stop path to explain why stop must happen before replacement.

Depends on : Sebane1/RoleplayingVoiceCore#8