THREE STOCK CAMPAIGNS -> MULTIPLAYER

UI Interface Editor destination list now contains only:
- Shadows of Amn -> Multiplayer
- Throne of Bhaal -> Multiplayer
- The Black Pits II -> Multiplayer

Planetar remains a separate START entry control and is not counted as a campaign choice.

Each stock campaign button now does:
1. Select that stock campaign.
2. Switch BG2 to multiplayer engine state.
3. Invoke the native Multiplayer handler.
4. Open CONNECTION_CREATE directly (Create Game).

It does not send the user to START_MAIN or the public Join Game list first.
